Don't get too caught up in splitting hairs over fund fees. Who's your broker? They all offer tools that can help you pick the best funds for your goals.
VOO is an ETF and VFINX is a mutual fund. Both of those are good choices, but understand the difference between the two. I think Schwab lets you buy partial ETF shares, but most do not.
As I replied previously, it's not all cut and dry when you need to withdraw contributions from a Roth IRA. Comparisons of a Roth IRA to a savings account are very misguided online. You'll have to sell whatever you're holding in order to withdraw the 'contribution'. There are some exceptions, such as first time home buyer. As a disaster fund, sure it could work, but everyone's situation will vary.
https://www.schwab.com/ira/roth-ira/withdrawal-rules
Broker: Schwab
Fund differences I've read about
EFT
Buy/sell any time market is open
Low expense ratio
No initial minimum investment
Taxes: going into Roth so not paying too much attention
Can buy fractional share
No trading commissions
Bid/ask spread (choose one with high trading volume)
Can take advantage of a dip
Mutual Fund
Buy/Sell at end of day @ NAV price
May require initial investment amount
Could automate contributions
May have higher expense ratio (unlikely if following an index)
Taxes: going into Roth so not paying too much attention
Set it and forget it approach
Trading commission fees (depending the chosen fund)
No bid/ask spread issue
Withdraw: I get that if I invest the money on the IRA any future withdraw is the sell of shares. So as long as I don't tap into any gains (to avoid penalties/taxes) and the market is + when withdrawing (selling), then it should not be a problem. It normally takes 3 business days to withdraw funds but given I will still have a savings account for emergencies it doesn't affect me if is 1 day or 1 week, as I'm not counting on needing immediately the money I put on the IRA. But I do prefer the option that if I have $300K (random number) in an IRA (w/ gains) and $150K are contributions and if those $150K can cover my expenses for 1,3,5 years I have the option to tap into that and call it a day a few years early than waiting until I'm 59.5 years old. I prefer to invest and get 6% return than leaving my money in a Money Market account at 1% which at the day I will be able to tap into the bigger gains
C fund: Would comparing the annualized rate of return from VOO (example) to the C fund be a good or fair comparison? or is more than just the return rate?