The Kone is a good technical upgrade over the 518 and the buttons are in familiar places. However, Roccats build quality and customer service is terrible.
Razers build quality is better, but only just. If you need an upgrade for MMO purposes then you should probably go for the Naga.
Although, I fail to see how the MX518 can cause you problems that aren't fixable by contacting people here or at Logitech HQ. Sounds more like you just want to buy something new.
Well if it's faulty I'd still take them up on the replacement and sell that on the Members Market. People will bite your hand off.