I'm not familiar with the Vanguard Employee Benefit Index, but if you're going to put money into a S&P 500 index fund, it's a no-brainer to pick the one with the lowest expense ratio. You couldn't get much better than an expense ratio of 0.01.
Regarding your question about how to allocate contributions among funds, it really depends on your goals, your level of knowledge about the various choices, and the beliefs you have formed based on that knowledge. If you don't feel like you can make an informed choice, a lifecycle fund might be the best way to start (that's a fund that allocates among various index funds based on your age and/or projected time to retirement). Fees would be a little higher that way, but it's probably better than making uninformed guesses about how to allocate your money. At your age, it would probably put you in something like a 90% stock 10% bond mix. Not a bad way to go.
Personally, I think investors at any age/stage benefit from some diversification among US large caps, US small caps, and developed international stocks, with anywhere from 10 - 40% diversified corporate and govt. bonds thrown in for ballast. The percentage in bonds really is based on your tolerance for big crashes in the stock component. In the early accumulation phase, I personally would not go more than 10% bonds, but I've got the stomach for riding out crashes.