The difference is that the "16" gear has a compact double chain ring crank on the front, and the "24" gear has a triple ring crank. Both versions will have a (probably the same) 8 cog cassette on the rear.
Don't think of it as 16 vs. 24 distinct speeds, as several combinations of front and rear gearing will give you virtually the same gear ratio.
Unless you live in a very hilly area, you'll do fine on the compact. The specs say that it has a SRAM PG 850 11-28t cassette.
Just as an FYI, you'll probably be better off posting your first questions in the Beginner's forum. :wink: