I've read that the driver inside the 555's is the same as the driver inside the 595, and that the only real difference is that piece of foam inside the 555's.
I own the 555's, and they are an excellent value for the price. If your ears aren't enormous (like mine are) they are probably the most comfortable headphones you can buy, with velor-esque material everywhere they touch your head. Even with my big ears, they are still comfortable for up to a couple of hours before I'd have to take a break. I don't use them much anymore, since I have superior headphones, but they were a great entry level phone.
Honestly, if you think you'll upgrade sometime soon, don't get the 555's though. There is no upgrade path for them, because they don't really benefit from an amplifier (in my experience anyway). If you're like me, you'll quickly be on your way to bigger and better headphones.