[FirestormX]

I have both, and personally I like the VFG on my shorter MP5, and the AFG on my longer M4.

The idea is that you want to have as much contact with the front of the actual gun as possible, so that you can have more control over it. For example, if you swing to your left, you want your gun to stop exactly when you stop turning. You'll have more control to stop the momentum of the front of the gun, if your hand is in the path of movement. A lot of people wrap their hand all the way around the barrel, for even more control.

For me, a foregrip is a good point of reference for where to put my hand, and makes the position of my hand a lot more comfortable.

When I hold my MP5, I'll wrap my pinky, ring finger, and thumb around the VFG, and extend my middle and index finger straight out along the barrel. It lets me hold it comfortably, and gives me a fair bit of control over a light and recoil-less toy.
On the other hand, a longer gun is harder to control (going back to the momentum scenario, a longer gun, means more momentum at the end of the barrel), and I like to have my entire hand around the barrel, instead of just my two fingers. The AFG is pretty low profile, and keeps me from having to twist my wrist a whole lot.

You don't necessarily need a foregrip though. I have an M4 with a rail set that's thick, and the AFG is good for that, but I've got another that has a regular set of rails, that (with a rail cover) is comfortable to hold with no foregrip. I'll run my MP5 with no grip half the time as well.

Go with whatever's comfortable for you. The good news is that foregrips (knockoffs, anyway) are fairly cheap, so you can experiment with them all you want. Pickup an angled foregrip, a vertical foregrip, a stubby...
Or see what other people are running on theirs at a game. Some people might even let you borrow theirs for a quick game during a skirmish, so you can see how it feels on your gun.