Magazine capacity should never be considered on AEGs. There are almost always many different mags that can be bought, usually in these classes: Realistic (20-30 rounds usually) capacity, low capacity (50-90 usually), mid capacity (120-200 or so), high capacity (250-800), drum/box high capacity (upto several thousand).
Batteries should be considered, but not that much either. Large type batteries are generally available in more power. Don't worry about the listed voltage. Pretty much all AEGs are usually ran at 8.4, or 9.6 for a higher firerate, or upgraded internals. One can go higher then that though, if needed.