It's probably just a preference depending on how large a squad you want and whether you want to be firing tons of poison or darklight/disintegrator. I run only 1 Raider, and that's just because my Grotesques can't ride in anything else...
I suspect 10 Wracks w/2 Liquifier Guns could put out a world of pain with Liquifiers alone, but I'd rather have more Venoms so I stick to a gang of 5 with a Liquifier. I think the Venom is just straight up a better vehicle, and unless I really want to transport more than 5 models in a single transport I'm happy with them.
The fact is, you're Flat Outing right up into your enemy's lines, and unless you have the opportunity to do that and be totally out of LoS your transport's probably gonna be taken out, so I'd be sure to consider how many Wracks you want to do the job.