I've played vanilla marines for 1.5 years. I have all of the heavy models. While I love Vindis and Preds, the best overall SM heavy unit IMHO is the Devastator Squad. The best value heavy support unit is the TFC, but that is another story.
The main reasons to go with a Devastator squad are survivability and consistent ability to shoot lots of heavy weapons turn after turn. No other heavy support unit I play can shoot 4 weapons every turn, and stay alive for many turns, if not the whole game. I often use Devs with the MoTF. He fits perfectly with my 10-man Devastator squads. I put him with them (combat squads) in a multi-story ruin, armed with the conversion beamer. Now I have 5 heavy weapons in two 2 and 3 weapon units, and they all get a 3+ cover save (bolstered ruin). Very, very effective and hard to kill. The main weakness is if assaulted. In big or C&C games I'll put a tactical squad and/or dread on the 2nd or ground floor to intercept any assaulting units. The only real drawback for Devs is that in a DoW deployment they won't be able to shoot the first turn. In my club we all hate DoW, so except in tournaments I rarely have to deal with that limitation.
My favorite load outs are as follows:
1) 10 men with four plasma cannons. Despite occasional overheats, four plasma shots, even if some scatter, put a nasty dent in any infantry unit or MC. Combat squads optional. MotF adds conversion beamer blast & range, and a 3+ cover save for the unit(s).
2) 10 men with two LC & two plasma cannons, combat squads. Good balance against unknown or balanced foes like SM or Chaos. MotF nice here too.
3) 10 men with two ML & two plasma cannons, combat squads. Like #2 but a bit cheaper and more flexibility with the ML's (frag or krak).
4) 5 men with four missile launchers. Cheap krak/frag spam.
Yes, they're an expensive heavy support choice, but 'you get what you pay for'. My gaming experience to date tells me that 40K games, all else being equal, go to the side that shoots/assaults the most. Anything that delays this (e.g. reserves) or otherwise reduces the amount one can do (moving, shaken/stunned anyone?) is a big problem. Devs shoot a lot, and for a long time. Other heavy units seldom do (TFC notwithstanding)