I currently have a Garmin 2730 but I'm thinking of upgrading. Is it better to buy a new "not refurbished" Zumo 550 for $500 or buy a Zumo 665 for a few hundred more? I already have the GM30 XM puck that I can use with the 550. The 665 comes with it's own XM puck. Will I be sorry if I don't get the latest and greatest?