I am now playing a stealthy (guns) sniper, and have to decide just this, at lvl 12. It is a difficult choice.
Some immediate thoughts:
Pros of Ambush:
-With fire grenades, or a flare at point blank, you can create ambush points in dark areas
-100% crit chance is doable in mid game
-Works with sidearms too, so you can use other guns if needed
-Can be used every turn, even several times
-Also havles enemy evasion, very helpful
Cons:
-Fire grenades can miss and mess up your plan
-Some areas are fully lit up, or unsuitable
-Require space to move, useless in a brawl
-Useless against crit immune enemies
-No benefit from smart damage modules
Pros of Snipe:
-Crazy damage with high stealth
-Require no A-A scope, so you can get a regular w/+10% presicion instead
-Benefit from smart damage modules
-Works against crit immune enemies
-Good with all sniper types
Cons:
-Require stealth (not being undetected, you can restealth in combat and snipe next turn)
-Has cooldown
But how do we play in practice?
I play a mobile stealthy sniper with decent dex+agi, traps, Quick Tinkering, Sprint and Strafe. So high mobility, rapid Spearhead, hit-and-run-tactics, traps a vital part of both attack and defense. For this Ambush is great, really great.
Unfortunately, Ambush doesnt always work, despite creative solutions. And when it fails, you are in trouble, usually with close range enemies breathing down your neck.
But Snipe also benefit from for example Blindsiding. And we have Aimed Shot too, and need to throw grenades and nets, quick tinker traps and move around, so many shots will be special damage shots too, to benefit from smart module and smart lenses.
Ultimately, I can not choose, it is too hard. I take Nimble instead to have more stealth and evasion.
But later, I want both.