Why wouldn't you put them both centered on the wall?

You don't say what size fireplace you are getting but once you get the firebox installed and stone/tile around it you will probably close to 7 feet for any reasonably sized fireplace.

If I did the fireplace on the side with very little tile around it, I would use the other side for a tall shelf and still center the TV above the FP.

Personally I don't think putting a TV above a heat source is a good idea, but that probably depends on how much you use the fireplace.

If you have the fireplace picked out or any drawings of what you are thinking about, post them here so more people can help you. The more details you can offer the better feedback you will get.