There is only one escort model at tier 2 that you can get, so there isn't any real choice. There are technically 3 "classes" of tier 2 escort, but the classes differ only in appearance; there is no functional difference between them.

You get to choose one new ship of the appropriate tier for free at each promotion, so you do not have to worry about saving up for a new ship. You only have to spend money on ships if you want to buy another ship of the same tier.