You actually have to ride 13 times to make the 7 day unlimited card worthwhile. Because of the 5% bonus when loading $5.50 or more in credit on a pay-per-ride card, you only have to load $31.45 (technically $31.43, but they only let you buy in nickel increments) on a pay-per-ride card to get 12 rides.

If you're really looking to extract maximum value, a pay-per-ride may prove to be the better bet (especially if all three of you are sticking together at all times and can share a single card, saving another $2 in new card fees). But if you're only in town for 4 days and plan to primarily use public transit to get around, having unlimited cards will give you lots of flexibility and free up your mind to worry about other things rather than keeping track of the balance on your Metrocard.