generally leds are better but its also dependant on brand.. at least 100 hz.. also look for new models that supports internet browsing, most video formats and flash player if possible..
Agreed. Don't buy anything that isn't 1080p and the motion looks awful on 50Hz sets, IMHO. Best with something at least 100Hz frame rate, more if you are sensitive to poor motion and flickering, and more so the larger the screen is. Watch them in-store rather than relying on the specs as it also depends how well it handles constructing the missing frames.
LED backlit sets aren't a whole lot different to conventional LCD sets. Screen technology is largely the same with a different type of backlight, so not surprisingly there isn't really a night-and-day difference in picture quality, IMHO. Some are good, some are awful with narrow viewing angles and poor blacks. They do tend to be lower power consumption and physically thinner if you're thinking about wall mounting. Display can be a bit brighter too. Certainly go for LCD/LED over plasma unless the required size forces you down the plasma route.
Online connectivity, etc. depends if you want everything in one box. If you just want a wall mounted TV with no additional boxes it can be handy but if you're also attaching a PVR, Blue-Ray player, games console etc. most of them offer online functionality so you won't need it in the TV.