2s1p would probably be a better configuration as it is 2 cells in series, giving the 7.2v. 2s2p would be 4 cells, 2 of each being in parallel and 2 in series, giving the same 7.2v. I would go the 2s1p as most batteries are like that, but I would think 2s2p would be slightly larger in size?
I have the Protek 5000mah 30C battery and it lasts a hell of a long time in my 21.5