I recently went through same dilemma. I eventually settled on a 10" Hitachi sliding compound miter saw. I have used some 12" miter saws in the past, but they are heavy and a lot of them have a pretty substantial kick when starting (which annoys me). My 10" Hitachi starts smooth as butter and I can cut through anything with it.
Yeah, if I was a framer and gonna be cutting 2x12's all day, I MIGHT use a 12" saw. But I truly believe a 10" is best for most folks.
Before purchasing mine, I did a Google search on "sliding compound miter saw reviews" and studied up a fair bit. After deciding on a saw, I looked in local Craigslist and found a gently used one WITH a super-duper Dewalt stand - total cost $400. I am simply thrilled with my purchase. It is a pleasure to use a smooth, accurate saw.
Good luck in your purchase. Do some homework first. Don't buy a piece of junk. As the old adage says, 'the quality is remembered long after the price is forgotten'.