Keyboard and mouse with mouseover macros. Aside from the obvious benefit of not needing to click on a target before healing them, it also allows you to heal while DPSing more easily since you don't have to reacquire your DPS target every time you need to heal somebody.

I tried the controller briefly and while it was alright and nice for playing while leaning back in your chair questing I don't think I'd enjoy it much for regular play. As a control scheme it doesn't really bring any benefit over KB/M (aside for the "comfy" factor) so I'd rather just play with that anyway.