The thing about Eldar snipers is that they have a 1-in-6 chance of ignoring armor saves. None of the IG snipers have that. - and it is armour that prevents snipers from being very effective against the big baddies.
Here's a little comparison - average number of unsaved wounds inflicted in one shooting phase. BTW, 10 ratling snipers and a lascannon heavy weapon squad cost exactly the same number of points:
Target Stat upgrades 10 Ratlings 3 Lascannons
Wraithlord 1.39 1.00
Carnifex T7 2+ sv 0.69 1.25
T7 3+ sv 1.39 1.25
Carnifex/Tyrant T6 2+ sv 0.69 1.25
- with 6+ inv 0.69 1.04
T6 3+ sv 1.39 1.25
- with 6+ inv 0.69 1.04
Some trends: the lower the save, the better the lascannon does vs. the sniper rifle. As well, the effectiveness of the lascannons drops if the target has an invulnerable/cover save.
Given these numbers, Ratlings do have a role against 3+ sv baddies. Against 2+, however, you might was well leave them at home. Personally, although the lascannons are easier to kill, I would sooner take them, as they are far better against vehicles and they can be used to autokill some of the smaller multi-wound cretures.
Inquisitor Psychologis Ruminahui