Gary,
Were I you, I would go right for the 2557. And, there are some reasons. The 25XX has a better receiver and a better routing algorithm. The larger memory also allows more other data to be stored.
I get asked this sort of question a lot. Maybe because I used to carry a job description of "Navigator". We have had more than a few for personal use. Not counting marine and air specific, that makes about five over the last 15 years. Some just got outdesigned, one died and three are still in service.
I have a 52LM, and by their specifications would stay away from the 55 as the routing algorithm is not that good. It also has no coverage of Canada or Mexico (if that matters).
We also have a 2595LMT. This is a much better unit. The traffic function is problematic as few places that we traffic have the service available. When it is, google maps often knows about it first. I have never even tried out the Bluetooth interface.
The oldest unit is so old that flints are hard to comeby (oops - wrong technology). It does still work and serves well for Geocaching, but not over the road.
Matt