Lactose: Is it about being sweet, or is it about the mouth feel?

I'm putting together a milk stout recipe I've been talking about with my brother that has a nice chocolate-coffee sort of thing going on.

I would like to do two beers based on this chocolate-coffee flavor idea. One that is a nice moderate beer and another that is more like an Imperial stout.

I've never used lactose before and I'm wondering if the use of lactose is more for the mouth feel or for the sweetening aspect of it?

Yes.

both. Milk stouts are also referred to as sweet stouts. For my next milk stout, I think I am going to go for 5% lactose and 5% maltodexrin as to get the really big body/mouthfeel/viscosity without too much sweetness.