While a good plasma is still the better looking product out there, depending on the room you are putting the TV into, you may want to opt for an LCD.
LCD, whether it is standard, or LCD backlit (in a couple of versions!) has seen significant improvements in recent years and is typically a good bit brighter than your typical plasma TV. In a dark room, it can actually appear to bright, but in brighter rooms, like open floor plans with large windoes, the LCD can be a great way to go.
I would recommend Panasonic for plasma... But, Samsung has some great product out this year which closely competes with Panasonic.
For LCD, I would stick with Samsung. I'm very impressed with their 8000 series units and haven't had issues with the ones I've installed.
Typically, people go to small for their main family room display, so if you are thinking 55", you likely could get away with a display over 60" pretty well. But, this is depending on viewing distance more than anything else. From 12' - about a 60" display is the smallest I would recommend.