Lethality/Dirty Fighting isn't inherently more complicated than Marksman/Sharpshooter imho. Well, I only ever played Dirty Fighting on my Gunslinger and I really don't find it very complex. Vital Shot, Hemorrhaging Blast, Shrap Bomb, Wounding Shots - everything else is basically filler, you just need to pay attention to your energy levels and use Aimed Shot, Sabotage Charge etc accordingly. Sharpshooter's energy management is even harder though, isn't it? I'm not sure about that.
Energy regen is not that big of a problem since staying in cover gives you +1 per second at all time. Keep followthrough (trick shot) pretty much on CD and alternating in 1 auto attack per snipe/charged burst rotation you can go on forever. Most new guys are easily panicked into spamming sniper/charge burst and kills energy.
Another reason I think marksman is easier is because ballistic damper its beastly defensive everytime you go in cover. Its a 30% damage reduction to ALL damage for 3 charges once every 6 seconds. It's definitely more newbie friendly imo.
Most newbie sniper wont know what to do when they get force charged. And lethality is very susceptible to force charge and rely more on player skill in maneuvering their toon. Also lethality has a lot of dots, not many newbie would trade in their 5k ambush/aimed for dots.
IMO lethality is definitely more advanced sniper spec. If you ever do 10-49 PvP you will see about 90% of the sniper/slingers are marksman/sharpshooter.