The Critical Pack is definitely an awesome pack to get. I definitely like it better than the ones with velcro since it makes it a little bit simpler and faster to load pods on and off.
The only con it has is its durability. I find that a critical pack is not as well stitched as a NXe or Empire pack. Although if you don't mind hand washing your Critical Pack then it will definitely last much much longer.
Other then being cheaper, you can manually feed paintballs with a Z2 using the rip drive... Handy in case your loader fails on you (rarely) during the middle of a game. I would also say that the Z2's feedneck is a bit stronger then the Spire just because it is thicker but don't take my word for it... Testing should be done to examine the durability of feednecks.
Everything else the Spire works better... Easier disassembly, feeds better, does not jam, speedfeed does not accidentally pop open during play, and a true 200 capacity. Did I miss anything?