I own an ATV but haven't really used it for hunting big game. For what you intend on using it for I don't see a problem with getting one. I don't think the cost of a new ATV would really justify saving some wear and tear on your truck but it's a good excuse if you are interested in getting an ATV. I use mine for plowing snow in the winter and for several camping trips throughout the year with my family/kids. I was seriously looking at the Yamaha's when I bought mine, and they are great ATV's, but I ended up going with a Suzuki Kingquad as it was a lot less money for what I felt was at least as nice an ATV as the Yamaha's. The Suzuki motor is noticeably more quiet than the Yamaha's, but again, any ATV is still pretty loud.
As far as your question regarding the 550 vs. the 700 it depends on what you want to get out of it. A 550 will get you anywhere you care to legally go on an ATV but there are a few situations where you can get there faster on a 700 so if that is worth the extra cost to you then go with the 700. If you plan on doing any long rides or have small kids on with you I'd highly recommend looking at a power steering model. I can't count the number of times I've had my kids fall asleep on a ride and I'm basically driving with one hand. The power steering is a life saver in those situations. It also helps absorb bumps that want to turn the wheels.
I've read through the thread and recognize there is some controversy here, and in large part I agree with it. Still, IMO, if used correctly they can be a good tool for hunting. I've just chosen not to really use them that way personally. Regardless, I think hunters in general are few enough and our rights are being threatened enough that, as long as they are doing it legally, I don't care how someone else chooses to hunt. Getting more hunters out is good for the longevity of the sport. I sometimes worry that my kids won't have the opportunity to hunt when they get older because of all of the political garbage happening. If someone wants to responsibly and legally use an ATV for hunting I have no problem with it. Besides, that limits where they can go and opens up some of the best hunting in the backcountry to the few of us willing to hike out and work for an animal.
Sorry for the long post, I'm done...