I've only just got my IA Sniper to lvl 14 last night, but i have 2 lvl 50 Gunslingers that i have recently been trying different specs out on.
So... it depends
On how you like to play. Mostly static then marksman, some static then engineering, mostly mobile then Lethality.
To be honest I do not think it makes much difference pre lvl 50, my first GS i lvl'd as sharpshooter(marksman) the second as Dirty Fighting (Lethality). Both are viable but do have slightly different play styles.
My currently played GS is Sab spec'd (Engineering) but I think I'll see how SS (MM) is now with the damage increase added in last patch.
Anyway, all my own opinion of course, but I wouldn't worry about it too much pre lvl 50, just try them out and see how you get on.
Oh, and I think you're on ToFN? If so, feel free to give me a wave, My IA is named Jaxal
Thanks for that mate. Just got to level 15. I might just stick with Eng then and move to leathality later, my play style is more on the mobile side but I like the look of the Eng tree with all the different probes etc.
Saying that I'm trying to get my Assassin to level 50 so will not play the IA that much. I just like mixing it up a bit.
I am on ToFN, I will add you tonight.
Assassin - dirkadirka
IA - durkadirka