The 2557 LMT means Lifetime updates for the maps, and Traffic updates too.
So if you are going to a strange town, it will be handy to have traffic ahead of you. I have a 6" screen Garmin with LMT and use it daily going to and from work. It displays where the traffic is heavy, and when I need to get off the freeway to avoid it. . .
It is also very helpful when getting off the freeway in finding new ways to get home. I can go down a empty side street, and it will update, showing the shortest way home from there.
I also enjoyed my DeLorme Topo program. I also bought the USB GPS receiver, and it plugs into my laptop, so I could plan routes several ways, and check the elevation changes with different routes.
Check the screen sizes, and go with what you need.
Check out this 6" refurbished unit. $120 from Amazon.com
http://smile.amazon.com/Garmin-Navigators-Directions-Preloaded-Refurbished/dp/B00TQYCWTI/ref=sr_1_25?s=electronics&ie=UTF8&qid=1450328092&sr=1-25&keywords=garmin+gpsThey have great prices on several units. Larger display is very important when in a RV, and you need to see it at a further distance than a car windshield, that is much closer to your face.
Good luck,
Fred.