Polaris 600 is a twin cylinder liquid cooled motor. The 550 is a fan cooled setup and limited on power. The 550 also has shown problems getting enough fresh air to keep it cool and is known to burn down due to this flaw. My wife (5'4" 130#ish) rides a 2004 600 RMK 144" and that seems to be a great setup for her. The sled is stable enough to not be spooky on the trail, yet be light enough in the front end it doesn't work her to death in the deeper snow. I like the bigger footprint of the 144 as she and I both pack a kid with us for now. She can creep around in the deeper snow without getting stuck too often. We used to ride with a lady that had the 550 fan/136x1.25" track. That sled disappointed me because it didn't have enough power to pull much of a hill or keep up on the faster road sections. Granted, it was light, but I still wouldn't want one. Plus, the wife's 600 makes a fun back up sled. I would recommend a 2003 or newer 600 RMK with the 144" track for your wife as this is a small block motor which I prefer and it will come with the PERC (electronic reverse). If the sled does not come with electric start, it can be added.
