I use chips and chunks both. The chips always fit and smoke more quickly. The chunks are sometimes too big and so have to be cut/split down, and take longer to get smoking good, but then the smoking last longer since they're bigger. That way you can spread your smoke out over a longer period of time using both.
Congrats on your MES! Don't forget to season it like it says in the manual before you use it for real. It'll do some mighty fine stinkin' during that first burn-off!