A lot of people have hinted at it but there is a few crucial differences between them, IMO. They are both the same type but they are very different Pokemon trying to do very different things; they don't really compete for the same role.
Thanks to have a total defense score of like eleven and a decent attack score combined with Pure Power, Medicham is obviously designed to kill Pokemon in one shot or close and die in as many hits. It(along with Heracross) are probably the two most obvious candidates for Choice Scarf, and using Medicham without that or Choice Band is kind of silly. It's very dangerous with both, and that's Medicham's game, coming in a few times on predicted switches, hitting hard, and getting right back out.
Gallade does it a little differently. While a little weaker than Medicham, it still hits very, very hard, much harder than most people seem to realize, but the main strength with Gallade is that due to the good Special Defense(even if the HP is pretty terrible), it can take special hits from a handful of Pokemon, allowing to come in more frequently than Medicham could, allowing to get more hits, even if each hit does less damage. Gallade obviously also has the ability to inflict pretty much every status effect in the game on the target if it desires, giving it a bit more finesse than Medicham tends to exert.
Different game for each of them, both great Pokemon.
