As mentioned, investor shares and admiral shares are substantially identical, but once you hit the minimum for admiral (usually $10k) you're automatically upgraded to the lower expense ratio.
If you're looking at Vanguard, for your funds, you will find very few differences. ETFs have the same expense ratio as admiral shares, and Vanguard doesn't charge any fees or commissions to buy/sell its own ETFs.
Personally, I hold mutual fund shares rather than ETFs. The biggest difference to me is that you can't buy/sell fractional shares of an ETF. So if an ETF is trading at $20, and I want to invest $50, I have to buy two shares, and hold the remainder in cash. While you can acquire fractional shares through Vanguard from re-investing dividends, you generally can't sell fractional shares. So if you need $35, you'll have to sell two shares, and keep the leftover in cash. Mutual funds will let you trade down to the .001 of a share, so you can always buy/sell the exact dollar amount you want.
The Bogleheads have a good page on the differences:
https://www.bogleheads.org/wiki/ETFs_vs_mutual_funds