I have had all three at the same time and it was an awesome lineup (Although my MET was a PRO).
I still have the AC/DC and MET. I sold the Tron only because I needed cash for a new car and it had the most value and quicker sale potential. I would still have the Tron and would like to get one back if I can at some point.
If I had to only choose 1 it would be Metallica. For one they are my favorite band. The ramps are awesome. Gameplay and rules are just killer. Sparky rocks. Its METALLICA! Fast and brutal games.