I compared a 10 man guardian squad w/scatter laser to one with a starcannon and then added a warlock with conceal and a spear (And another guardian) to each to both a 6 man las/plas squad and a 4 man termie command squad w/2 AC's.
Interesting results on a (points killed/points shooting) / (Points lost/Points shooting at unit) scale-
Vs the Tac squad-
At Long range, the 10 man Starcannon came out on top with a 3.00 Rating. The Starcannon w/ Warlock was second at 2.45
At Medium range, the Starcannon with warlock came in first with a .47 rating. The Scatter laser squad was second with a near identical result.
At Short Range, the Scatter laser squad came out on top with a .96 rating. The Starcannon was second with a .89
Vs the Terminators
At Medium Range the Star Cannon w/warlock came out on top with a .62 rating. Regular star cannon was second with a .61 rating
At Short Range, the Star Cannon w/ Warlock came out on top again with a 1.70 rating. The Scatter Laser w/ Warlock was second with a 1.61 rating.
In 4 out of the 5 situations, a starcannon was the still the best weapon. But looks like, in this case, there is a use for both weapon platforms.
Of course, it also shows that Guardians definitely don't want to be in bolter range, but out of catapult range.