I'm not really too up on the details or differences ( aside from price )
My 50'' Sony DLP went out today, again... went out around 8-9 months ago and i had to replace the bulb. Rather than replace the bulb and go a week without TV, i'm going to buy another!
well.. .mostly because I got a tax check a week or so ago so i have money to buy one. The TV that went out was my wife and my first tv. We got it when we moved in together around 2 years ago ( used ). Picture has always been really good so I want to get something at least the same size.
I found this samsung set which is 780p. after 20-30 minutes of comparing bestbuy, sams, costco, walmart, and target i went to amazon and found this set. $799 + free shipping and no tax since it's amazon. Bestbuy actually has the same set at a local store but there is $65 in tax added to the price.
Since your bulb is going out frequently on your DLP, I'm going to assume that you have your contrast settings pretty high? If not then ignore the following. [Even though I'm a hardcore fan of Plasma.] You may want to consider a LCD instead of plasma. While plasma has the best detail, motion display accuracy, price, and richness of colors and blacks over LCD, it does lack in backlighting capabilities in high ambient light viewing areas and suffers more from glare.
Samsung is a great brand to go with, so you're on the right track. Most of their products are tried and true. Panasonic is comparable, but a little more pricey. Anything else is a gimmick IMHO (especially Sharp, Vizio, LG, and Insignia). That's coming from about 6 months of researching almost every day.
-720p versus 1080p-
The real question is the media you'll mainly be playing. If it's Blu-ray, then yes, you want 1080p and a 46"+ set. If you don't plan on getting Blu-ray within the next 3-4 years, or playing 1080p content from a computer, the extra money for a higher resolution and display set isn't worth it. With a native resolution of 1366 x 768 pixels for that size of television it will be highly noticeable if you're within 5 feet from the set. So consider that.
For the size of 50", it's almost definite you will want to have 1080p, but that's only in combination with a higher native resolution. The reason why the set you're seeing there is so cheap in comparison is due to the fact it has a low native resolution for it's size. While it doesn't completely negate 1080p content, it makes a qualitative difference in display. Native resolution is a huge driving factor in price when it comes to HDTV sets, so keep that in mind and you'll see it, especially with cheaper brands.
Amazon is the best way to go if you don't have to pay the tax on the set and get free shipping. It can end up saving you upwards of 15% as opposed to buying in a local store. However, I've found that HHGregg is pretty reasonable for price matching and their selection is fairly nice as well. I really wouldn't bother with any other retailers.
Additionally, if you end up getting a plasma, one of the main things to know about them not commonly spoken is there's a break-in period that you should utilize full screen displaying for it to allow the phophorus in the set to mature. I've seen recommendations of 150-200 hours of a broad range of colors and motion on a medium contrast and brightness setting to insure the life of the set.
I hope this isn't overwhelming you with information, but rather that it helps!