I did some diving a few years ago there. I went with Abracadabra
Aquaventures (Ed and Jeanie Comfort). Awesome diving, the reefs are pristine
and the Grotto dive was way cool. I didn't get the opportunity to dive
Tinian. Truk is out of this world. I did a week with the liveaboard Odyssey
last year. I cannot express how great the diving was there. The crew and
boat were most excellent.  Enjoy!