For those too lazy to use the search button (or Uzi's handy dandy links), here is a quick down and dirty on the Stranger-Fex, the VC-Fex, and BioVores. To figure Double weapon Fexes, just double my hit/miss numbers (% will be the same)
Hitting:
The BS Fex will miss 50% of his shots. 1 Shot a turn = 1 hit every other turn
The VC Fex will miss 50% of his shots. 2 Shots a turn = 1 hit every turn
The Biovore will never "miss" his shots, he just might not hit immediatly. Missed spores remain a threat and move randomly until destroyed or detonated.
Wounding:
The BS Fex will wound most enemies on a 2+, they get to make their saves. It will glance weak armor fairly easily, tough armor you'll need 5s or 6s (1 shot a turn, you do the math)
The VC Fex will wound, at worst, on a 3+ (if anyone knows of a critter with a 10 T, let me know). SM and better will get their armor save, Tau and worse will just get splicked. It will glance weak armor on 2+, and tougher armor can be glanced 50% of the time.
The Biovore, using poison, will wound everything 50% of the time, regardless of who you are shooting. SM and better get a save, Tau and worse choke to death.
using BioAcid, the Vore has a chance to pen even the toughest of armor, but it being so random (where it lands, where is shoots, what its strength is), its not very reliable.
Cost:
The Biovore with mines is the most expensive, the VC is next, and the BS is last. The BS is the most cost effective of the three, but you get what you pay for.
In summation, use the VC against vehicles and the Vore with Poison against troops.