Just to be slightly pedantic, pet 'Charge' isn't a stun. It's immobilize. Basically a 1 second root. This means it won't interrupt spellcasting.
Charge
That said, I actually tend to spec both, but keep one or the other turned off. They don't share a CD, and by having both, I can make the pet 'dash' at one target, and 'charge' another. Usually I have Dash off autocast and leave Charge on.
If you need to choose one or the other, I'd want to know what you're using them for. Because for me, of the pair, even with the stun and bonus attack I would prefer Dash for PVP and probably go Charge with raiding/instances. Grinding and leveling... it doesn't matter as much unless you want to maximize DPS even then.
Charge is a 'one and done' speed boost. When you're in range of the individual, you'll zip after them. If someone is moving fast, you may not get within that range to use it. However, Dash lasts 16 seconds, and can be triggered out of combat. This means if you need to move fast for reasons other than 'getting to the target', the pet will be able to catch up. Thus, good for PVP in particular. Even with the pet's stun used by charge.
However, another argument for Charge: it works through stuns and slows. Dash speed is still limited by these things, while Charge, if your pet can get within range before it's slowed, will fire off, letting them zip to the enemy in the same speed. I noticed my Devilsaur demonstrating this in BWL.