If I get to have a Sniper Rifle, or at the very least a scoped rifle or a scoped assault rifle with zoom, im good.
Oh and silencers and/or suppressors to... lots and lots of silencers and/or suppressors!
My playstyle is basicly the same with all modern and/or sci-fi games where firearms are available... I sneak around close to all the time (be it necessary or not), observe the enemy and/or the area, and often for huge amounts of time.
I look for the best ways to move through an area, where all the available entry points are to buildings or areas, where goodies and loot might be... and I especially observe the enemies patrol patterns.
I figure out where the best place is to take out each individual guard as silently as possible, so that no alarms are raised, and once I am done with my scouting I then decide if I need to snipe at long range (often due to no ability to take them down undetected, few snipers can equipt/come equipt with silencers/suppressors, atleast in early game, so it's mostly going to be loud), or sniped headshots at short/point-blank range with silenced weapons (like rifles, assault rifles (usually set to single shot), or powerful enough handguns)... sometimes I will take them down with melee, but it's rare and mostly if for some reason using my firearms is not optimal.
I rather take 10-30 or more minutes scouting the area and taking down the enemy silently, then go all Rambo spray-n-pray and be done with it in a minute or 2. One shot, one kill, at the right time, is what I go for most of the time.
Strange thing is that in Fantasy games I never go for the ranged characters. In Fantasy games I am totally the heavily armored (plate armor if available), one handed sword (I ALWAYS use Swords in fantasy games) and shield wielding, guy... who sneaks around all the time... XD Which is not all that different from Sci-Fi games... except the weapon part... ranged rather than melee, rifles rather then anything else.