Depends on TL (atleast if you use HG). At TL12 or less, power plant output is relatively low and installing lasers on the single turret of your small ship will slow its down (lose Agility, in game terms); missiles are generally better at lower TLs as they are prime ship-killers (especially if you manage to get your hands on a nuke or two). At later TLs, your powerplant can handle the lasers, and then their advantage of not needing (cargo-space consuming) ammonition is worth it.
So at TL12-, one sandcaster and two missile tacks; at TL13+, one sandcaster and two beam lasers.