Honestly, an xpen is easier than NIC grids IMO, and less expensive. Kieko has a NIC condo (4 wide x 4 tall x 2 deep), and it can’t be taken out of the room without taking it apart, and we spent 4 times as much on her condo as we did on an xpen.
The pros of the xpen: easy to change shape and relocate, built-in walk-thru door with latch, no zip ties to have to make sure are pointed outward when setting the xpen back up after relocating, harder for bunny to climb; cons: if you buy one that’s too short you have to replace and can’t add to the height.
NIC grid pros: can be added to at any time, great for blocking off areas especially small spaces or odd shaped spaces. Cons: must use tons of zip ties to make sure it is sturdy, while zip ties last well there are always some that break, must make sure zip ties are pointed outward so bunny doesn’t get scratched or must smooth each after cutting.
I know there are lots more pros and cons to both, but these are the ones that came to mind.