If you intend to do the PCT next year with one of the three packs that you have listed, I would recommend the ULA Circuit.
It is, very likely, the most used pack on the PCT. Last time I posted a question about this on the pct-l mailing list, I would say that well over 60% of the people who posted were ULA users, with the large majority of them using the Circuit.
The Circuit provides the necessary amount of room for the two stretches of (average) 12-15 days without resupply, provides bear canister room, and provides some of the best compression straps out there, for when you do not need all of the extra food and canisters and water.
The Circuit will also be the kindest to your body for whether you have a base pack weight of 8 pounds or 28 pounds.
The Ohm, will tend to falter on you if you are at the upper level of the typically accepted PCT PBW levels - remembering that at least two times you will probably have nearly 30+ pounds worth of supplies.
Perhaps you could share what your existing BPW is and what you are using for your big three, so we can better help determine how much room you might need. For example, a 32 degree bag verses a 20 degree bag. A tarp/tent that uses hiking poles or a tent that requires folding poles (and thus more room taken up inside your pack). How much cloths you plan on taking and how much mass they take up. All these kind of factors would really be helpful in narrowing down the option for a pack. Chances are you could maybe pull it off with a 3500 cu in pack, or maybe you will need a 4000+ cu in pack, or, maybe, you can make it all happen with a sub-3000 cu in pack. All really depends on the rest of your gear. As the old say goes "you buy your pack based upon your gear, not your gear based upon your pack".
Its just hard to know without a more detailed gear list. But if you are just looking for us to pull a name off the top of our heads based on overall experiences, I would suggest the Circuit.
I have owned the ULA Circuit, ULA Ohm, HMG Windrider, ZPacks Blast 26 cf, and ZPacks Zero xsmall cf - and without a doubt, the most comfortable pack in that list is the ULA Circuit. Amazingly comfortable.
For my planned 2012 PCT trip I plan to use a modified ZPacks Blast 26 cf, with a planned BPW of around 5 pounds. I just do not need the extra space that the ULA Packs have, nor all the comfort features (read: extra weight) that the Circuit/Ohm offers... but man, are they nice!! Sometimes just thinking about the ubber comfort of the shoulder straps on the ULA Circuit makes me want to carry more gear - giggle. Oh, and lets not forget the new 'female' version of the chest straps that ULA offers... I list myself among the men out there that wish more backpacking manufacturers offered those... man did ULA nail those!! Unbelievable comfort, even for us guys.
If you are not already apart of the pct-l mailing list, and you intend to hike the pct next year, you really should join it. It has more pct alumni than probably any other venue.
Anyway Casey, some further details on your existing gear list would be really helpful!!
John B. Abela