Lock up is great in both. The new design in theory will be slightly better but I would think in practice makes little difference. The low rider clip is nice though on the new model but with the great deals to be had on the old model you can't go wrong with either. The cut in the inside makes for a nice smooth uninterupted back scale but the recess can collect a bit of pocket lint that you can't just wipe out, you can easily blow it out though. This in no way impacts knife function and you won't see it so unless you are totally neurotic about blowing lint out of the inside really doesn't make much difference. Both are very nice knives.
The problems you have heard are vocal minorities who have that 1 in several thousand that has an actual issue, which ZT/Kershaw fixes quickly, or people that have disassembled their knives and created the issue themselves because they don't know what they are doing. ZT/Kershaw are quick to take care of these issues as well.
If you don't own a ZT or Kershaw yet you may not know, but their customer service is second to none. I recently asked for a old style ZT 550 clip and they sent me an entire parts kit with washers, frame screws, the whole works. They don't get enough praise here and sometimes it may seem like there are lots of issues but that is far from the truth, the only reality is that there may be a lot of whiners. Get a ZT, either new or old. And if you don't absolutely love it, you should have no problem finding someone who will take it off your hands at or near what you paid for it. I think once you have a 550 in hand, or most any ZT for that matter, you'll love it and there will be no need to worry about what price you'll get for a used ZT.
