Go for the isku if you want, the merc stealth is an old product

But, what's your budget range? And what kinda games do you usually play?

On that range, you could get the Logitech G510 as well but seeing that you are currently using roccat mouse, going with the isku might be a better idea. I'm not too sure if roccat has their hard wares integrated when used together but if it does then that will add more functionality to your peripherals