The Exos 58 - but I'm biased.

If you can, go to the store, load your gear in, and try them both on. I looked at the Z55, but it was awkward (for me) to pack, and I couldn't get a good fit; it was very close, but no cigar. The Exos 58 worked very well on both counts. Aside from the great fit on the suspension, what really sold me was the two vertical outside pockets: I could store my fly in one, tent inner in the other, and have a wet fly separated from a relatively dry inner; the poles store in whichever pocket balances the load from side to side. Storing the tent in the outside pocket has two other advantages: it lets me pitch the tent without opening the main pack in the rain, and it lets me pack inside the tent, then take down the tent and pack it without having to open the main pack and expose it to the rain.

If you can't get to a store, and have room on the credit card, order one of each pack from a dealer that allows returns of unused gear for a full refund. Leave all the tags on, and load in your gear, then walk it around the block. Send the one you don't like back.