So this question is kind of like asking, I have a ST1100 that I like, should I upgrade to the new Goldwing?
The 550 was the most reliable GPS that I've owned. I've owned the the 590, and the BMW Nav 6 before my purchase of the XT. THE 590 and Nav 6 left me disappointed. They worked, but not without issues. I've only had the XT for a year, but I'm very happy so far. It is a very different beast than the 550. There will be a learning curve.
If you pair the XT with a smart phone using Garmin Drive the GPS will preform like Google maps. It will alert you to traffic issues or detours and recommend detours. This is really helpful if you are routing multiday rides.
Having used both I can highly recommend the XT. Especially if you can purchase it on sale. This usually saves you $100.