I'm not sure if they came out with the 550 tundra in 06 or 07. I owned an 08 Tundra 300F that I put a ton of miles on. Same basic motor as in your old Tundra but with a few tweaks. Yes the front end is heavier, makes them steer like a dream (possibly not so much with the bent ski). There was a lot of talk about the front suspensions on them not being as tough and they are all exposed. I bashed mine off trees, rocks, gates and anything else that happened to be close to the trail and never had a problem. The front suspension setup made them steer great, and I would take that over any theoritical weakness that I was never able to discover myself. The one thing I will say bad about the exposed steering and coilsprings is that if you are boring through brush and tag alders it is alittle bad to catch a limb or tag alder in it and either jerk you sideways or bring you to a sudden (as in over the handlebars if you aren't paying attention) halt.
IMHO you want the 300F not the 550. The 550 is a good machine, gets around good, runs for a long time if it doesn't blow up in the first 500 miles (they had some issues with that, but should all be resolved in any used machine, never heard of one having a problem if they made it past the break in) but they burn so much fuel that you can't carry enough with you to make a full day if you are breaking trail or riding in the rain. And will drive you broke if you don't own stock in Exxon.