me and my group are looking at the sniper rifles and really can't figure out why one woudl use them over the stalker pattern bolter?

Off the top of my head…. the sniper rifles are available earlier, and you don't have to requisition special ammo for them to be fully effective. The Stalker is more of a DMR, anyway.

Renown requirement plays a role, and the slightly better range of the sniper rifle may do so as well.

That said, once you get your hands on the Stalker, the sniper rifle is clearly inferior for most assignments. Which makes sense, as it's a weapon for scouts, not full-fledged marines.

Now, the Ultra-pattern sniper rifle from Rites of Battle is much, much better, actually a viable alternative to Stalker for actual sniping (as opposed to tactical marksmanship, in which Stalker is still superior).

Sniping in general just seems pretty silly in DW. You're just better off being with the team anyway, and are more of a liability off on your own. You don't get squad mode (unless you're with a spotter), its highly possible that terrain/buildings will limit your visibility to the target area, once the enemy determines there is a sniper, they can plan accordingly, etc. I get that its a play style some players might think is cool, but in practice, it will leave you bored at the table.

As far as the Astartes Sniper Rifle vs. Stalker pattern… I would say it more readily accepts toxin use, while a bolter would require a test. The Astartes Sniper Rifle is really meant for specific targets, mainly unarmoured ones.

Sniping is good for Wolf Scouts.

Sniping is excellent for Kill-Marines.