If an infantry unit plans to sit in place, adding a trio of AC's to 3 units Infantry Squad units is dirt cheap compared to a HWS with the same loadout, 40% of the cost, to be exact. They're such a cheap upgrade, for so much potential, that I would put them in squads I planned to move
just in case I had the chance to fire them... if I had the points to spare. I'd sooner put them into a moving squad then put them in a HWS.
An autocannon adds some anti-infantry punch to a squad, which is always nice, but most importantly it adds the ability to open transports. IG aren't the only armies going mechanized, after all, and if you have no infantry to shoot at, a pair of AC's and a pair of GL's gives you something with reasonable range and punch to take out a Rhino / Razorback... which gives you infantry to shoot next turn.
Once my infantry squads are full, I'd consider buying HWSs. The thing I don't like about them, is that they need a CCS within order range
and the need to be deployed within a Lord Commie's Ld bubble to make them worthwhile. In particular, without a LC's Ld, they have terrible Ld on their own. What's more is that after a piddly 2 wounds, or a single ID wound, you have to take a Ld test.
3x Lascannon HWS = 105 pts. Avoiding their LOS isn't super hard on a typical board.
They take two wounds, on a Guardsman's profile, and then have to take a break test. They fail 41.7% of the time and will probably run off the board if they fail, since they're always deployed at the "back" of the line.
When they fire at a vehicle, they typically generate one or two hits in a round.
They pass a BID order a mere 58.3% of the time on their own. If they pass, they typically generate 2-3 hits in a round. High strength is good against High AV's, but melta is much better.
A 5 man Storm Trooper Squad with two Meltaguns = 105 pts. Deep Strike in, Avoiding their LOS is nearly impossible.
They will get to fire, for sure, before being forced to take a Ld check. When they're fired upon, they fail 27.8% of the time, and when they fall back they may be able to rally on their own, and if not they may fall towards a CCS that has the potential to get them "Back in the Fight". Even if not, your enemy will either dedicate further resources to klling them off, or stay outside of 12" of them.
When they fire at a vehicle, they typically generate one or two hits in a round.
Melta hits are much better than LC hits, and are thus much more likely to damage whatever they're targeting.
Against any kind of mobile army, they'll just start peppering your HWS with small arms fire till they run off. Then all your big guns are gone, and you have a bunch of flashlights. Keeping HWTs inside squads boosts their survivability substantially. Combining a blob with a Commie means your orders are more effective [remember your Special Weapons benefit too!] and they're much more likely to pass... especially with a single Vox in there. They will pass an order roll 97.2% of the time, and are only 2.8% likely to fail any morale check.
I'm a junkie when it comes to Autocannons. I'd have one for each Infantry Squad you could take.
I'd make a total of 3 Lascannons [I like them in my CCS squads, and sometimes in a PCS squad, or you can join them all up in a HWS if you really want to...]
I like to put a Missile Launcher in with a trio of Plasma Vets... so if you take those I'd have one for each. Otherwise 3 for a HWS, as they're quite flexible without being super good. This tends to help lower their position on an enemy's target priority list.
A Mortar HWS avoids most of the pitfalls of the other weapons. They're out of LOS, so are hard to target and thus don't take damage, so they don't need to pass morale checks. They don't benefit much from orders, so don't need to worry about passing a check. They're cheap. Because they're out of LOS, they can be sneaky objective holders, and are unlikely to be an easy KP in other missions. That said, they are mortars.
They're cheap for a reason. 3 for a HWS.
I don't like Heavy Bolters in squads, because they're not much better at AI firepower than an AC, and they don't give you the same options vs light to medium vehicles that an AC does. That said, and I can't recommend this all that strongly, If you're ordering a blob to BID with their AC's, it's nice to have a unit with ranged Anti-Infantry firepower to follow up with, after all the orders are done. I prefer a LRBT with Plasma sponsons, but if you've already got the HB's made, a HWS worth of them is great for firing into ejected passengers, light infantry in the open and MEQ in cover. [They get their save anyway, so save the plasma for guys in the open!]
EDIT: For the same price as an AC / HB HWS, you could buy a Hydra. For 15 ponts mor than a Mortar HWS, you could have a Griffon. I already mentioned about Stormies and Lascannon HWS.