I wish the mmo 7 had some real buttons and not that multi direction thing on the thumb side. I would totally get one.
Find a mouse that is comfortable for you...
I don't find the idea of having all abilities on the mouse all that great mostly because of my style but that shouldn't limit yours.
The mmo mice aren't very good sensor wise (don't know about the corsair one) the naga will jump (always down and to the right) if you are the type to lift your mouse up at all. But that doesn't seem to bother people.
As someone who played WoW for 5 years and spent the majority of that time being in the guilds that raid 5 days a week and aren't quite as good as the best (with the exception of one that raided 3 and got pretty high in rankings), I found that everything I needed was almost easier via the keyboard and the mouse was better used like an extra hand (for checking the environment, for targetting and for healers; hovering over unit frames) just keeping it doing it's job and not much more. I would use the side buttons for rare use abillities sometimes or targetting macros or that silly shield paladins had at one point. I also used the mouse buttons as heals via the addon clique at one point.
Sure I played with a lot of pro-naga people paticularly in more casual guilds (When I was tired of the whole grind) and yes it probably suits 1 armed WoW players better (yes there are). I can't stress enough that at the end it's about being comfortable, if you are really really hardcore the absolute best is to develop your keybinds to relate to each other; e.g. stun on 5, incapacitate on shift+5 and so on and so forth given that you have with some rebinding a minimum of 18*3 possible binds give or take a few.
So take what suits you comfort wise because nothing else matters. The steelseries and naga mmo mice are fairly big but I think theres a slight change to size in one of the naga builds but can't remember if that was true or not..