I have both. Perhaps there are other factors but my 720 is not as good at my 1080 but the 720 looks damn good.
What is most important is the "FOR THE MONEY" factor. First off the tv is going to be better than what you had and everyone is different.
Make sure you have HD box and have the HD service. 720 is going to look great.
Im really into this stuff and help my friends build systems. Fun hobby.
There are no real answers. Side by side you'll notice a difference but my 720's in HD look good.
So if they are giving it away, go and get it! The only thing is 1. Budget and 2. will you be happy. You can always spend more and always be happier.
These things are easy to cross shop on spec. You have to compare plasma to plasma (still best bang for the money) vs LCD's vs LED's. Big thing to look for when watching sports is the refresh rate. Especially on basketball. Slow rate on a dirt cheap LCD or LED you will see whats called "ghosting" or "pixelizaton". Plasma's are less prone to this as the refresh rates are higher. Most 720's have lower refresh rates. Its a pirce thing.
Don't know where your shopping or your budget or how big, or how big your room is. I'd sacrifice size for quality. Smaller with clarity is always better than big and blurry. Go with plasma if they still have them. Problem is most stores have stopped carrying them.