My Standish Standoff list contains a full unit of Devastators. Every Devastator unit has to include the obligatory sergeant, and I kept mine bare bones. Bolter, bolt pistol, and signum.
While cleaning parts and deciding on a pose for my Devastator sergeant, last night, I got to thinking about all the possible armaments and load-outs this one guy can have.
On the surface, a Dev sergeant is identical to a sergeant in a Tactical squad. You start with the typical Marine stat line, a bolter, bolt pistol, frags/kraks, and power armor. However, the Devastator sergeant gets a signum, which allows one model in his unit to fire at BS5, so long as the sergeant is alive and does not fire in the Shooting phase himself. It's a very handy tool.
The Dev sergeant is able to purchase any upgrade you can typically purchase for a Tac sergeant. Ranged weapons like pistols and combi-bolters, as well as melee weapons like power weapons and fists. You can also take the Veteran upgrade, as well as meltabombs.
In my opinion, the first thing you have to decide on is Veteran status. Taking the upgrade gives you one extra Attack and one point of Leadership. So, two attacks base instead of one, and Ld9 instead of Ld8. I don't feel an additional attack in melee is useful for a model in a shooting-dedicated unit. The additional point of leadership, however, is useful.
Devastators are typically a backfield unit. They have a nice, long range band with all of their weapons, which means they're often sitting near your back table edge. This position means that failed Morale tests are dangerous. Lose two or three Marines in a turn and you're risking falling back off the board. Even if you don't fall off the board, the involuntary movement means you lose a turn of shooting as you regroup and regain your fire lanes (unless you are running Ultramarines Chapter Tactics and burn your Devastator Doctrine for the turn).
Meltabombs have always been an upgrade you take as a safety net or last-ditch option. They're cheap enough to throw in without thinking too hard. For Devs, they don't make quite as much sense. You can' throw meltabombs, so they're only applicable in melee against a monstrous creature or a vehicle. Most MCs that charge into a unit whose sergeant has meltabombs tends to challenge out that sergeant to remove the bombs entirely (either by killing the sergeant with AP2 wounds or forcing the denial of the challenge). Walkers are seriously threatened by meltabombs, but if anything but a drop podded Dreadnought ends up in close with your Devastators, I think you may have done something fundamentally wrong that meltabombs won't fix.
That brings us neatly into the melee weapon options. Every one of these falls under the same logic as meltabombs and the Veteran upgrade: Devs aren't a melee unit. If they find themselves in a fistfight, something has gone terribly wrong. Do you really want to spend the heavy points investment in something like a power fist that you may not/probably won't use?
By my thinking, the only viable options besides Veteran status are the ranged weapons, as they match the role of the unit. But which one to take?
I'd write the pistol options (grav and plasma) off immediately, due to their 12" range. Both pistol types are too expensive to take as insurance, single-shot weaponry. I'd skip the combi-flamer and combi-melta for the exact same reason, though they are less expensive as insurance.
A combi-grav is a neat little gem, but its max range is only half that of the weapons it most closely shares a target priority with (plasma cannons, fired at medium and heavy infantry). It does put out a solid three shots when stationary, and the signum is nearly useless for plasma cannon Devastators. I'd say a combi-grav is an "ok, not great" choice.
All that's left is the storm bolter. It's an Assault 2, 24" range weapon, and is as cheap an upgrade as you can get. That range band is at least useful for some of the heavy weapons it'll be alongside. When the unit packs multimeltas, you'll want to use the signum. Plasma cannons make the storm bolter shots into an afterthought, but more dice rolled for shooting is never a bad thing. Lascannons are like multimeltas, where you want the signum instead. Missile launchers split the difference, where they want the signum if you're firing krak/flakk, but the storm bolter shots if you're firing frag. Your closest relative for the storm bolter will be heavy bolters. If you calculate it all out, upping a single heavy bolter to BS5 over BS4 will net you about the same number of wounds on Toughness 4 models as using the storm bolter instead. The difference is only about .30 wounds in favor of the storm bolter.
After all of that rambling, my theory is that there are only two real choices to make when kitting a Devastator Sergeant: veteran or not, and storm bolter, combi-bolter, or regular bolter.
Vet status depends on what other Leadership mechanics are in your army and where. For my Standoff list, I'm running Cato Sicarius, who gives a blanket Ld10 to your army, so long as he's alive. That makes Vet status moot for the Dev sergeant. I feel that if Sicarius is dead early enough for the Devs' Leadership value to matter, I'm on the downslope anyways.
The other option is a banner in a Command Squad or Honor Guard. Honor Guard can't be made into fire support units, so their banner probably won't be anywhere near the Devs. Command Squads can take a whole lot of plasma guns and become some sort of mid-range firebase unit, parked near the Devs, but it's unlikely.
The bolter-or-not-bolter question really depends on how you arm the heavy weapons Marines, and if you're a fan of small points spent on small probability wargear (the statistical chance a storm bolter by itself will kill a MEQ model is small, but when stacked up alongside other factors, it can add up).
Well, there's some mental vomit of "40k tactics" talk for you. I'm sure I've missed something in this pages-long post, so feel free to discuss the math, the options, and the situations for which you'd arm a Devastator Sergeant.