It REALLY depends on your play style.
Cruisers are more "Wear the enemy down because I'll survive"
Escorts are "Kill the enemy quick because if they all start attacking me, I'm not going to survive..."
Science Vessels are "HAHAHAHA! You can't do ANYTHING .... oh... but it's going to take me a minute to kill you... I'll enjoy watching you die slowly... MUHAHAHA!"
Does that help?
I always play damage class first...it's become habitual...so I went with escorts in beta and in live...
I am grooming my 'super-character' to be a cruiser captain though.... They seem a bit more interesting overall...so when the escort is where I want it to be, I'll immediately start moving the cruiser through the ranks...
That will be the ship that is purely there for experimenting and fun...
I still have a spot for science too after making an obligatory Klingon....so I'll go science there...just to round it out... =D
(still hoping for more diverse experiences with different ship types in the coming months)
Well seeing as whoever spams the most DPS, escort of course. As of now the idiots at cryptic (i.e. those who came up with the loot concept) have it so Cruiser and Science vessels, while very nice to help along mission completion, don't get any points for support. High epeen DPS = the most loot and best able to farm (and leaves one questioning why to roll anything else other than tact/escort other than the warm fuzzy and undergeared feeling playing a support or tank role gets you). Much the same as every Star Trek series and movie out there focuses on whoever kills the most gets the most glory, the game wishes to reinforce this motif. No wait . I think they saved some whales once, but my guess is that those whales were in fact killer whales.