If you tell us what else you have in your army, that would help us out a lot. That way, we can see what you've got in your army and see if those choices will help fill in a gap that your army lacks.
However, if you're into tourney games, I would suggest the Bassie, many tournements don't accept Forge World units (Becuase no one bloody knows if its legit or not) - and the Bassie is a good solid unit that is in the Guard codex. You'll get no qualms from that. The bassie itself packs a bloody big punch and can be used in either the direct or indirect role, it also has the whole psychological factor that it brings in.
But, if you don't play that many tourney, and you're just looking for something awesome - I would suggest the Heavy Mortars just on the "ohhhh cool!" factor. That, and they seem to be pretty decent on paper. But then again, 3 mini-grifs should allways be awesome.