Do you plan to try and market time, or for the most part buy and hold (2 three trades a year)?
If market time, disregard.
Based on the data below from TSP. S Fund is the best fund over the last 1 year, 3 years, and 10 years; however, F Fund out performed over last 5 years. Therefore, I would sit in the S Fund as much as possible. If I could get a loan outside of TSP for less then the S Fund return (7%) I would recommend not taking a TSP loan. With all that said monthly seasonalty states "sell in May and go away", and come back in October.
I'm a market timer and you can read more of my thoughts on that in my thread.
[TABLE="class: tspStandard"]
[TR]
[TD="class: subHeading bold, colspan: 11"]Average Annual Returns (As of December 2012)[/TD]
[/TR]
[TR]
[TH="class: leadingCell"] [/TH]
[TH="class: packed"]L Income[/TH]
[TH="class: packed"]L 2020[/TH]
[TH="class: packed"]L 2030[/TH]
[TH="class: packed"]L 2040[/TH]
[TH="class: packed"]L 2050[/TH]
[TH="class: packed"]G Fund[/TH]
[TH="class: packed"]F Fund[/TH]
[TH="class: packed"]C Fund[/TH]
[TH="class: packed"]S Fund[/TH]
[TH="class: packed"]I Fund[/TH]
[/TR]
[TR]
[TD="class: leadingCell"]1-Year[/TD]
[TD="class: packed, width: 9%"]4.77%[/TD]
[TD="class: packed, width: 9%"]10.42%[/TD]
[TD="class: packed, width: 9%"]12.61%[/TD]
[TD="class: packed, width: 9%"]14.27%[/TD]
[TD="class: packed, width: 9%"]15.85%[/TD]
[TD="class: packed, width: 9%"]1.47%[/TD]
[TD="class: packed, width: 9%"]4.29%[/TD]
[TD="class: packed, width: 9%"]16.07%[/TD]
[TD="class: packed, width: 9%"]18.57%[/TD]
[TD="class: packed, width: 9%"]18.62%[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]3-Year[/TD]
[TD="class: packed, width: 9%"]4.24%[/TD]
[TD="class: packed, width: 9%"]7.03%[/TD]
[TD="class: packed, width: 9%"]8.09%[/TD]
[TD="class: packed, width: 9%"]8.83%[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]2.24%[/TD]
[TD="class: packed, width: 9%"]6.29%[/TD]
[TD="class: packed, width: 9%"]10.90%[/TD]
[TD="class: packed, width: 9%"]13.92%[/TD]
[TD="class: packed, width: 9%"]4.13%[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]5-Year[/TD]
[TD="class: packed, width: 9%"]3.14%[/TD]
[TD="class: packed, width: 9%"]2.44%[/TD]
[TD="class: packed, width: 9%"]2.32%[/TD]
[TD="class: packed, width: 9%"]2.01%[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]2.69%[/TD]
[TD="class: packed, width: 9%"]6.06%[/TD]
[TD="class: packed, width: 9%"]1.71%[/TD]
[TD="class: packed, width: 9%"]4.22%[/TD]
[TD="class: packed, width: 9%"](3.30%)[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]10-Year[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]3.61%[/TD]
[TD="class: packed, width: 9%"]5.25%[/TD]
[TD="class: packed, width: 9%"]7.12%[/TD]
[TD="class: packed, width: 9%"]10.79%[/TD]
[TD="class: packed, width: 9%"]8.39%[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]Since Inception [/TD]
[TD="class: packed, width: 9%"]4.17%[/TD]
[TD="class: packed, width: 9%"]4.82%[/TD]
[TD="class: packed, width: 9%"]4.95%[/TD]
[TD="class: packed, width: 9%"]5.00%[/TD]
[TD="class: packed, width: 9%"]5.81%[/TD]
[TD="class: packed, width: 9%"]5.69%[/TD]
[TD="class: packed, width: 9%"]7.01%[/TD]
[TD="class: packed, width: 9%"]9.50%[/TD]
[TD="class: packed, width: 9%"]7.12%[/TD]
[TD="class: packed, width: 9%"]4.06%[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]Inception Date [/TD]
[TD="class: alignRight packed"]08/01/05[/TD]
[TD="class: alignRight packed"]08/01/05[/TD]
[TD="class: alignRight packed"]08/01/05[/TD]
[TD="class: alignRight packed"]08/01/05[/TD]
[TD="class: alignRight packed"]01/31/11[/TD]
[TD="class: alignRight packed"]04/01/87[/TD]
[TD="class: alignRight packed"]01/29/88[/TD]
[TD="class: alignRight packed"]01/29/88[/TD]
[TD="class: alignRight packed"]05/01/01[/TD]
[TD="class: alignRight packed"]05/01/01[/TD]
[/TR]
[TR]
[TD="class: subHeading bold, colspan: 11"]Calendar Year Returns[/TD]
[/TR]
[TR]
[TH="class: leadingCell"] [/TH]
[TH="class: packed"]L Income[/TH]
[TH="class: packed"]L 2020[/TH]
[TH="class: packed"]L 2030[/TH]
[TH="class: packed"]L 2040[/TH]
[TH="class: packed"]L 2050[/TH]
[TH="class: packed"]G Fund[/TH]
[TH="class: packed"]F Fund[/TH]
[TH="class: packed"]C Fund[/TH]
[TH="class: packed"]S Fund[/TH]
[TH="class: packed"]I Fund[/TH]
[/TR]
[TR]
[TD="class: leadingCell"]2008[/TD]
[TD="class: packed, width: 9%"](5.09%)[/TD]
[TD="class: packed, width: 9%"](22.77%)[/TD]
[TD="class: packed, width: 9%"](27.50%)[/TD]
[TD="class: packed, width: 9%"](31.53%)[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]3.75%[/TD]
[TD="class: packed, width: 9%"]5.45%[/TD]
[TD="class: packed, width: 9%"](36.99%)[/TD]
[TD="class: packed, width: 9%"](38.32%)[/TD]
[TD="class: packed, width: 9%"](42.43%)[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]2009[/TD]
[TD="class: packed, width: 9%"]8.57%[/TD]
[TD="class: packed, width: 9%"]19.14%[/TD]
[TD="class: packed, width: 9%"]22.48%[/TD]
[TD="class: packed, width: 9%"]25.19%[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]2.97%[/TD]
[TD="class: packed, width: 9%"]5.99%[/TD]
[TD="class: packed, width: 9%"]26.68%[/TD]
[TD="class: packed, width: 9%"]34.85%[/TD]
[TD="class: packed, width: 9%"]30.04%[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]2010[/TD]
[TD="class: packed, width: 9%"]5.74%[/TD]
[TD="class: packed, width: 9%"]10.59%[/TD]
[TD="class: packed, width: 9%"]12.48%[/TD]
[TD="class: packed, width: 9%"]13.89%[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]2.81%[/TD]
[TD="class: packed, width: 9%"]6.71%[/TD]
[TD="class: packed, width: 9%"]15.06%[/TD]
[TD="class: packed, width: 9%"]29.06%[/TD]
[TD="class: packed, width: 9%"]7.94%[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]2011[/TD]
[TD="class: packed, width: 9%"]2.23%[/TD]
[TD="class: packed, width: 9%"]0.41%[/TD]
[TD="class: packed, width: 9%"](0.31%)[/TD]
[TD="class: packed, width: 9%"](0.96%)[/TD]
[TD="class: packed, width: 9%"]-[/TD]
[TD="class: packed, width: 9%"]2.45%[/TD]
[TD="class: packed, width: 9%"]7.89%[/TD]
[TD="class: packed, width: 9%"]2.11%[/TD]
[TD="class: packed, width: 9%"](3.38%)[/TD]
[TD="class: packed, width: 9%"](11.81%)[/TD]
[/TR]
[TR]
[TD="class: leadingCell"]2012[/TD]
[TD="class: packed, width: 9%"]4.77%[/TD]
[TD="class: packed, width: 9%"]10.42%[/TD]
[TD="class: packed, width: 9%"]12.61%[/TD]
[TD="class: packed, width: 9%"]14.27%[/TD]
[TD="class: packed, width: 9%"]15.85%[/TD]
[TD="class: packed, width: 9%"]1.47%[/TD]
[TD="class: packed, width: 9%"]4.29%[/TD]
[TD="class: packed, width: 9%"]16.07%[/TD]
[TD="class: packed, width: 9%"]18.57%[/TD]
[TD="class: packed, width: 9%"]18.62%[/TD]
[/TR]
[TR]
[TD="class: subHeading bold"]YTD[/TD]
[TD="class: packed, width: 9%"]2.11%[/TD]
[TD="class: packed, width: 9%"]5.00%[/TD]
[TD="class: packed, width: 9%"]6.28%[/TD]
[TD="class: packed, width: 9%"]7.23%[/TD]
[TD="class: packed, width: 9%"]8.07%[/TD]
[TD="class: packed, width: 9%"]0.38%[/TD]
[TD="class: packed, width: 9%"]0.01%[/TD]
[TD="class: packed, width: 9%"]10.61%[/TD]
[TD="class: packed, width: 9%"]13.09%[/TD]
[TD="class: packed, width: 9%"]4.34%[/TD]
[/TR]
[/TABLE]
https://www.tsp.gov/investmentfunds/returns/returnSummary.shtml