I was looking at same thing, have an older zumo 450 currently. Left my Garmin power cable on my other bike when I traded it in, should of took it off. Anyway debating whether to order a new cable or GPS (thinking the cable at this point).

One thing I determined is Garmin model numbers are very confusing I was actually leaning towards the 390, mainly for the extra battery time (7 Hours VS 4 Hours on the 590), I'm fine with the "bit" smaller screen, no MP3 player, there are a few extra "bling" features that I wouldn't mind having, but not sure I'm willing to pay $200 extra for them.

Here's a comparison, everybody is different. Think I'm going to stick with my 450 for a-bit longer, I really do like the lane assist feature of the newer GPS units and the lifetime map updates are really nice. My car GPS has the traffic updates which I do not see on either the 590 or 390, I would like to have that too..