Functionally, I would say they are about equal. The gen 2 0550's changed the pocket clip and lock bar but neither change is enough to make the gen 1 obsolete. I believe jimmer said that the lock bar change is an improvement, it's not to say that the gen 1's lock bar is bad. So, it does come down to whether or not you want deep carry or not. I think if you look on the Kershaw/ZT archived subforum, you can find a technique for retrofitting a gen 1 with a gen 2 clip, though I don't know how recommended that method comes, if at all.
Also, since the 551 was a limited run, you might want to keep it just because of that. One reason is from the collector's perspective but the other is that if you ever wanted to get it back, you'd have a harder time imo (though I don't know how you came to get the one you currently have).