One of the coleaders of Elysium (world firsts for raiding) plays on a gamepad, so I certainly wouldn't call it limiting. It is very unlikely to ever have a certain answer for which is the absolute best (you would need high end players to become totally proficient and both, then give opinions). However, either is clearly viable, so it's best to give both a good try and decide for yourself - the input method itself won't be nearly as much of an effect on your skills in game than other factors would be.