I own both the 212 Pros and the Sennheiser 555's. Since you are looking for a portable solution I would not recommend the 555's
They leak way too much sound and their form factor (too big !) doesnt lend itself so well to mobile listening
I recently bought a Sony Mp3 player and love it but just as I got to enjoy it, my old Sennheiser HD 212 Pro's broke so I'm looking to replace them. I liked the style of the 555's better but I liked the closed ear design of the 280's better for the portable purposes that they would be needed for....Any advice for me?