As you have to fully commit to Demo to use (effectively) the Felguard, I would say that your choice is entirely based on your own play-style.
Personally, the VW's torment has always been nice (I have imp vw) as I'm an affliction lock - he hold's aggro for 1/2-3/4 of the mob's life, at which time it's very easy to drain-tank/tap. You'll find that there are times (level wise), when his torment doesn't keep up with your dps, so you should modify your spell rotation to take this into account. It's also especially useful if someone tries to gank you - pop bubble, fear, coe, run - or fight). It also allows you to take out 6-7 mobs at a time (dot dot dot... iHoT... refresh dots... bubble... drain).
You also rarely have to spend downtime - as consume shadows or health-funnel (with the fact that you're taking little damage, and drain-hunting), leaves you and it in good shape for the next fight.
Dps wise... the vw is like a dot 10-15 levels below you. If you want pvp/survivability, Demo is a great spec; since you said you mainly pve, this isn't a spec that fits my playstyle.
Also, it'll suck having to spend money to continually respect (because few raid guilds prefer a demo lock over an affl. lock - excluding pvp where demo is more viable.