How about DLP?
Cheaper and longer lasting then both LCD and Plasma.

I purchased a 46″ slim depth 720p dlp projection hdtv Samsung would you say this was a good investment or should I purchase a lcd

Plasma has more contrast, slightly larger color space, deeper blacks and larger size than LCD.
But plasma show some degree of flickering. Go to a TV shop and you’ll see that, while looking at the plasma in front of you, you can sense the flickering of the ones on the left and right. The brighter the room, the easier will be to see the flickering.
LCD has longer life expectancy, consumes less power, doesn’t flicker (no need to refresh the pixels as they stay lit by themselves) but does not come in such large sizes.

Ok. opinions are like you know what. I have been in the industy for 10 years+. I have worked for several major companies. I have strong relationships with about 30 vendors and reps. So I will give you my 2 cents worth on the subject. Dlp DOES NOT last longer than Plasma or LCD. Here is the skinny long lasting in order to longest to shortest: plasma, lcd, rear projection (dlp, lcd, or lcos) Plasma tvs have been in production the longest. (since 1964) from IBM.
Lcds are great 40″ and smaller. Lcds are getting close to where they need to be. you still do not have color accuracy like plasma.
1- You have banding issues on lcd. and black levels are not black
2-plasma you have window/light glare and they can run hot.
3- Rear pros have less of side viewing and you have to replace the bulbs.

I know it is hard to read opinions and wonder who to trust. I have answered service calls on all 3 technologies over my 10 years. I have the least amount of problems with plasma. Plus the highest end tvs are Plasma (Runco, Fujitsu, Vidakron, Pioneer, NEC) And yes they do lcd as well except Pioneer but the bulk or their tvs are plasma. Plasma tvs give you the longest warranties: Fujitsu 3 years, LG 2, Samsung 2, Runco 2, Pioneer elite 2, Hitachi directors 2. Most important regardless of what tv you get I give you my professional advice to maximize you picture and lifespan:
1- get a power conditioner (Richard Gray, Panamax, Belkin, Apc, Furman, Monster)
2- get SATELLITE cable is not 100% digital and it is very compressed.
Direct-tv, Dish, AT&T, Verizon are the best
3- use good cables (don’t be cheap about this.) if a price sounds too good then it is so spend the money
4- most important get the tv calibrated (ISF) and no you cannot fully do this with a dvd.
5- just because the sales guy told you that you need 1080p because it is better than 720p/1080i dont’t believer him. broadcast will not be 1080p EVER. it is not physically possible.

I forgot… contrast ratio mean nothing. humans can only see 800:1