If its a "Sit and Shoot" until, go for the second heavy weapon. The extra firepower I think is worth it. If you don't wanna draw too much attention and want it to be cheap, you might consider a Mortar. I usually recommend against taking single mortars but if you have a very specific purpose for it (and the squad its in) it can work out. Like if you wanted toput Harker's squad really far out of the way as more of a Terrain Denial unit, then Sniper Rifles and a Mortar would work quite well.
If its going to be operated at shorter ranges but still immobile, a Heavy Bolter or AutoCannon is fantastic.
If you want it to be mobile, then I'd grab a heavy flamer and any other two special weapons you can fire on the move.