I'd definitely go with the Heat Ray. The other weapons are not nearly good enough to justify any points cost (there should almost be a points reduction for taking the TL HGauss Cannon...). A double shot multi-melta is a pretty good weapon to start with, and the flamer option is nice if need arises (overwatch...). And seeing how most of the Necron army is short ranged too, I don't think that the range of the Heat Ray will be much of a problem. I thought about using the Particle Whateveritwas, but with that it just seems like an overpriced Defiler with a worse gun and no CC ability. The re-rolls is what I'd use it for, that's a pretty powerful ability. Helps taking down those tough units through weight of fire. How has that worked out for you?