I've got the costco vizio plasma at my home and just bought one for my grandmother whose home is at 7000 elev.
The problem with plasma at elevation is you will hear a buzzing noise which is the fan (maybe scavaging for air). This did happen but it wasn't anything you would hear unless you knew about it and were listening for it.
I prefer Plasma over LCD, as I for some reason notice the pixle jump on the LCD tv's. The newest Sony has something that loads the picture twice as fast as a standard LCD, and for this reason they say that the pixle movement is no longer an issue (this is mainly for high speed sports programs). My father bought the 60" sony with this and I still can see the movement. The picture quality on this unit will blow you away.
Go down to Best Buy and talk to the TV crew. You'll be there for hrs, and get several different opinions, that are for the most part informative.
For brands, I'd look at the Samsung or Sony for the top, all the others being a step down, but comparable to each other. Brands like NewVision, Pioneer Premier being something that you don't even ask how much, its the best.