I've used both. I just finished a 5-day, 55-mile HOT, humid section of the Sheltowee Trace with the Z-55. I've used the Catalyst on a number of short trips since I got mine in February.
To cut to the chase, for the JMT (and I'm hiking it in July), the Catalyst is the hands down winner. My reasons? Several.
1) The suspension is simply superior IMO. The dual-tension hipbelt on the Catalyst is the most comfortable, securely fitting I have ever tried. The rest of the suspension is superb, with only one possible exception: it fits very close to the back and can get very sweaty in humid, warmer conditions. (This is the reason I used the Z55 and was very glad I did in the hot humid heat of Kentucky. Its bowed, ventilated backpanel was much appreciated. But its suspension is not nearly as comfortable with 30 lbs as the ULA Catalyst.)
2) With a bear cannister, the Catalyst truly shines. I can turn my BearVault 400 sideways and set it into place with Catalyst. With the Gregory Z-55, good luck getting a large cannister into the pack at all in a way that makes packing the rest of your gear anything short of a headache. You'll have to stand the cannister upright and pack around it and this can a pain with the reduced interior space due to its ventilated back panel.
The only issue in going with ULA is that if you want one, you'd better order it NOW. You should plan on a month's delay any way, and if Brian is having trouble getting some of the smaller more technical parts from his suppliers, you could wait longer. I had to wait about 7 weeks, but it was well worth it and Brian was great about letting me know why there was a delay and when to expect the pack.
Either way, good luck and have an incredible walk on the JMT.