Which ever fits your hand the best. Both are super reputable company's. Also Bravo Co, VLTOR, Daniel Defense, Magpul and other make really nice form grips. Some fore grips have storage in the grip, some are longer than others, some have retractable front mounts and so forth.
Fitting your hand really has nothing to do w/ it. You shouldn't be gripping it like a broom stick or can of beer. If you're gripping as such you are loosing a lot of accuracy and mobility. I would recommend a hand stop. Larue makes some that are really nicely priced.