Ed,
What weight rod are you thinking of purchasing?
I have had a 6wt Xi2 for two years now, and use it everywhere. I initially bought it for chucking big indicators with long leaders on wind blown interior lakes for big trout. However, I use it all over the place from fishing big dries/nymph rigs on big rivers to beach fishing for coho/pinks. My 690 Xi2 is definitely the most versatile fly rod I have ever owned and used.
I haven't had a chance to cast any of the new Z axis rods yet, but have waved them around a bit in the store and at shows. They feel very light in the hand and more faster than a Xi2. I have casted several XP's which have found that they are fast and load easily. I found that the Xi2 has lots of reserved power when your using the double haul, meaning it won't buckle under the most aggressive hauling and can really shoot line.
It's a tough call between the two. If your going to be strictly fishing lots of weighted flies, nymph rigs and shooting head type lines, than I would go with the Xi2. If your going for a more multi-purpose rod, thats able to fish both delicate presentations and long casts when needed, than go with the Z axis. The Z axis is available with the fighting butt and full well handles from 6wts and higher.