Jakewash is pretty much correct in his assessment. The big thing is to know where your going to put the TV, what you like to watch and how far you will sit from it. General rule of thumb is 2.5X the diagonal length of your TV is a safe seating distance. We now find the 61" small at 14 ft, so I would say go 50" if you can.
I bought a 61" Lcos based rear projection because it would be in a living room exposed to sunlight. If I got a plasma it would have a huge glare from the reflective screen, so putting it in the basement where there is not a lot of sunlight is perfect for plasma.
Plasma image retention is rarely an issue and only if you leave images on it for extended periods of time and dont break it in properly.
Both LCD and Plasma have great viewing angles,(projection does not) but generally plasma has a shorter response time of 4ms vs. some LCD which are 8ms.....that means you may see some ghosting on fast moving images, but many LCD owners dont notice that.
My personal preference for 42 or 50" is plasma. Panny, Pioneer and Hitachi are great brands and prices are coming down. for 37" or smaller, I'd go LCD. Bang for buck is RPTV(Lcos is the superior technology) but viewing angle is a drawback.
Good luck
