The "downside" to buying Devastator boxes for Grav Cannons (just doing the bits is usually not worth it, or they are recasts) is having a bunch of other heavy weapons left over.
I decided to go with a unit for Dark Angels, going with all Lascannon.
So this my next project waiting impatiently for my tax refund to begin a new, more focused army build. Right now the only thing I know I'm going to buy is a Dark Vengeance box. That will be a nice expansion to my CSM (mainly more cultists) as well as enough Dark Angels models to have a good sized force of those if I want to play them - mainly as a non-competitive Marine army.