You might also want to consider the Nexys2 from digilent. This is somewhere between the Basys2 and Spartan 3E Starter Board, both in price and features.
The Nexys2 has the same size fpga (500k gates) as the Starter Board. I guess it depends on what you want to do with it, now and later. If you are ONLY going to do the ADC thing and implement it efficiently then you can maybe get away with the 100k gates fpga on the standard Basys2. If however you see this as a board to experiment with, and the ADC thing is only the first of your experimentations, then you may want to get a board with a larger fpga.
I had much the same dillema a few years ago and decided to go with the Nexys2 and pay extra for the 1200k gates option. That way I could easily fit my first project in it, and later I had room for larger projects. If I had gone for a 100k gate device some of the later projects would not have fitted...
So I guess it becomes: buy the cheapest board that fits your current and future needs.
PS: Oh, you could try and find a place that still stocks the Avnet spartan-3a evaluation kit. That board had pretty awesome price/performance.