I like option 2 a lot better, strong enough to wreck transports and most tanks with a charge, more wounds with high T and can cause much more damage to large units, also they get to actually do a little bit of overwatch. Plus incubi are very vulnerable to anti-marine shooting which every one runs, just image getting hit with flamers of tzeach or a hell drake, where your grots would probably survive.
I do love the incubi, you just need a lot of them if you are going to kill a unit in one round.
In either case you should stay away from GK termies, 4 incubi most likely won't do enough unsaved wounds to prevent them from attacking back and wrecking your face with 2+ wounding via hammerhand and no saves (assuming you even get to go first!). Then on the grots, don't forget they all use force weapons and can just choice to activate instant death on you, turing those 14 wounds into 5 wounds with no FNP and you definately won't go first.
The worst sort of protection is confidence. The best defense is suspicion.