As BATTLEGUN stated, the 700 Grizzly is a more discussed machine on this forum. This is partially due to it's huge popularity when first introduced to the market. And it is a great all around machine. As for the 550 vs. the 700, I would take into consideration your needs. The machines are identical in size. Without looking at the specs of each, curb weight may be the only difference. If the 550 is lower in weight then it may allow better handling. But I think the biggest thing in which Yamaha was trying to do was offer an “entry level” vehicle at a lower cost without a lot of re-engineering. This is just a stab in the dark at an opinion. Do take into consideration that the larger displacement vehicles tend to yield a higher and quicker resale value.
One thing is for certain, and I feel many will concur that have experience. Do test the vehicles with and without the electronic power steering. The low failure rate on the power steering proves that it has been extremely reliable.
Good luck, ask any questions and welcome to the site.