The GT lets you monitor a bunch of stuff like tranny temp, AFR, throttle %, and more. You leave it mounted in the truck. One negative is thieves will steal the dam thing. That's what happened to mine :crying:
The pup has the same tunes but you just load it and remove.
If you watch the Buy/Sell threads on this site you can get used pups for about $200 shipped and used GTs sell for $300 shipped.