In reply to The Big Man: "i'm told that the 2 man can be a bit tight, but then perhaps it's better to have the lighter 2 man with you, than the heavier 4 in your car!"
If you are that bothered about the weight you could get the teranova untralight ones. As far as I can see from the ones we had as a walking club the ultralight ones are exactly the same as the older normal ones that didn't have windows and had a slightly thinner material.
Another advantage of this is that the windows can become brittle with age and split in cold weather.
I have a standard 4 man TN and I'm considering taking one of the the windows out.