I read your 1850 list from 4/27 wildeyedjester. Not having read the new IG codex, I can only guess what nastiness Vendettas and Hydra do. If my count was right I have you at 9 vehicles? *shudder* Vehicles are the bane of nids, and I have evolved my list for the Big Waaagh to address that risk. I would say my list is 90% solid and I have about 120 pts left to play with.
1 winged Hive Tyrant (utter killing machine, but tends to draw lots of low AP fire)
1 Hive Tyrant - Equipped with 2 sets of TL devourers (12 S5 shots reroll to hit AND wound)
3 Warriors - Leaping (12" charge), rending - good for hard units and vehicles
3 Warriors - Leaping, rending
3 Warriors - 2 with S6 blast, 1 with S4 ord
8 Genestealers - Scouts, rending, fleet, preferred enemy (down to one scouting unit cause they never arrive when I want them to)
10 Genestealers - Rending, fleet, preferred enemy (one of the best cc units in the game IMHO)
18 Hormagaunts - Leaping, fleet -
18 Termagaunts - fleet, S4 reroll to wound gun
8 Spinegaunts - without number, fleet (can you say meat shield?)
Carnifex - Venom cannon (2 shot, S10), Barbed strangler (S8 ord)
Carnifex - Same as above
Last choice is up in the air. I see benefits (and weaknesses) of each.
Carnifex - no guns, but extra attacks (4 S9) - run into enemy lines targeting tanks or tough units. Tough, but relatively slow compared to other units in army.
3 Raveners - taken as three (single model) FA choices to make enemy only able to target one at a time. Fleet, beasts, rending, deep strike - not that I would when they move that fast. 15 S4 rending attacks, good at catching fast vehicles/units, but (as is it with most nids) they are all offense and no defense and die easily to small arms fire.
9 Genestealers of 20 Termagaunts - good when engaged, but fragile to fire.
If the battle is close, my nid army tends to lose steam in turn 4-5. Not having a lot of resilient units is a bitch so I am leaning toward the third Carnifex.