If you plan to actually carry the backpack often, I wouldn't recommend the Porter 46. It has no frame, a hipbelt basically just webbing, and some half-hearted shoulder straps. It's an awesome pack for business travel, etc! But if you plan to keep it on your back for more than thru the airport to the taxi, etc, I'd look elsewhere.
The Jam or Pinnacle could be good. Also consider the Osprey Kestrel series, great value for a pack... maybe the 38L? Other considerations: new Osprey Stratos 36 has a panel load; Gregory Z65 has a massive panel load, and will compress down pretty tiny; Deuter Spectro AC 38 is another panel loader. Any of these would be better if you plan to keep the pack on for many hours a day.
I prefer a "one big tube"-style backpack for backcountry travel, but front country stuff is a lot easier if you can just reach in and grab a pair of pants w/o digging everything out of the pack... which is why I recommended several panel loaders.