Correct me if I am wrong, I am by no means trying to offend anyone here, but is there even a such thing as portable open back headphone? What do you mean when you say you want a headphone that you can take with you? Are you trying to use them on a bus or a plane? My experience with open back headphones is that they act more as a small speaker than a headphone. My two favorite open back headphones are AKG K701 and Sennheiser HD650. When I listen to them on a moderate volume in the same room as my family members for example, they can hear everything just as much as I can. In fact, if I take them off my head, I don't feel that there is much of a volume difference. The difference I can hear when I have them on are details, small nuances, etc.
Anyway, if you enjoy a warm sound signature and like planar magnetic, then both Audeze LCD 2 and LCD 3 will work for you fine. My experience with Foestex TH900 and Beyerdynamic T1 is that they are both wonderful sounding headphones, but they sounded a little bit cold and bright for my taste. But, that might just be me, because I am really sensitive to high frequency details. On the other hand, Audeze headphones have smooth, relaxing bass response that is never overpowering, nor bleeds into the mid. With most planar magnetic headphones, the mids are where the Audezes are at. Both male and female vocals sound seductive and celestial. Beatles sound phenomenal. The only downside though, is the fact that I can't imagine these being portable. They are open back, heavy, and take up quite a bit of space. If you are willing to wait, however, Audeze recently announced new EL-8, which is cheaper, lighter, smaller, and comes in both closed back and open back version. Good luck!