I take at least 2 plasma tanks on every mission when they become available. I use them to scout ahead of the soilders and when they find an alien posistion my troops so they can fire upon them. If the tank is destoryed, no big deal just build another, no significant lose of experience as they dont gain it. I would rather lose a tank to an alien attack than any man (usless they have v.low Psi strength). I have on a regular basis only used tanks to wipe out a crash site, there is no point risking men to kill a couple of aliens when all I want is the goodies. There are occassions when I have to use men such as assulting a base or large UFO (Single doors) but I keep the tanks close 1. for protection, 2. To take reaction shots from aliens. Using this tactic i can assult any size ship and only lose a few men around 5 at the worst guess. If i have Psi this figure is next to nothing as I take over the aliens, disarm them and then use men to gun them down.