Personally, I hate fees and investing in either the 500 OR the total market index will give you a huge amount of diversity, so I would just pick which one you like and go with it.
Also, from what I understand from speaking with Vanguard, once you set up your Admiral shares (at a starting price of $10k), they remain Admiral shares unless you cash out shares. As I understand it, it doesn't matter if the market drops and the value goes below $10k. They'll keep them as Admiral shares.
I know I'm in the minority, but I like the 500 over the total market index because it gives you slightly more exposure internationally, at the expense of small-cap stocks.. It's the large and medium-sized players that make much of their money internationally - think Apple, Exxon-Mobile, Coke, etc... the total market fund dilutes this somewhat by having a lot of small-cap stocks in there.