The Better Weapon?

[bad_humor]

Let's pretend I'm totally new and unexperienced in list building... 'cause I am.
I'm putting together a squad of tank-hunting tactical marines. Which is better for the special weapon choice, a grav gun or a melta gun?

[YorkNecromancer]

Meltagun for Tank-hunting.

Grav-gun for killing Monstrous Creatures.

But I wouldn't go tank hunting with Tac marines. If you need a dedicated anti-tank, a better bet is to save the points for a Sternguard squad - give all five combi-meltas and a drop pod. Combi-meltas are also very easy to convert from standard Bolters and Inferno Pistols, so you can do them on the cheap financially.

Unless you're doing HH/30K, in which case go Iron Hands Special Weapon Squad and give all the guys Graviton Guns (which aren't the same as 40K-era Grav Guns)- with Haywire, every Tank is toast.

[Kirsten]

I would agree with not taking anti tank tactical squads, though I would go for devastators over sternguard personally

[Denzark]

To answer your question - if you must use the Tac squad for this, the Grav gun can do a maximum of glancing hits on a six. The Meltagun can do a maximum of an explodes on a 5+ damage roll for a non-opened topped vehicle. So The answer to the question is meltagun, every time. As Yorkie says, to take out anti-tank many things can do better than tac squads with melta.