I spent a lot of time researching TV's last year when I was in the market to replace my outdated 60" projection TV. Here's what I discovered:
Plasma has the best picture, but lower life cycle. If you're like me and have the TV constantly on, you probably don't want to go plasma. Even though plasma has the best picture, pricing tends to be lower than LCD/LED of comparable size. The reason for this is plasma's overall life tends to be less.
240hz refresh rate is hardly discernable to the naked eye. You won't be able to tell the difference between 120 and 240. Therefore, it's not worth paying the extra $'s for a 240hz.
Most entry level LED's are backlit LED TV's. IMO, not worth paying the extra $'s unless you're going to get a pure LED system. These are pricy and tend to be upwards of $2,000 for a 50"+ TV. LED's advantage over LCD is energy consumption and width. The picture benefit of LED over LCD is negligible. You can achieve higher contrast and color dynamics with LED, it just depends if you want to spend the extra $'s. However, I believe the future will be LED and OLED due to energy savings and thickness. The market is headed in this direction, but pricing has not come down enough.
I settled on an LCD...Samsung 55" LCD LN55C630. The overall picture quality suits my needs which is primarily sports. You can get these at Best Buy for $1,199 and free delivery.
Best time to buy a TV is the week leading up to Super Bowl Sunday. Although, LCD's in general are being slashed due to oversupply. You can get a great deal on an LCD or backlit LCD/LED TV.
Good Luck.