I love the edger. It works great. I've never had it clog with the new design. The giant debris flap works great also, never get any mud on my legs anymore.
I personally like having a dedicated piece of equipment if i use it a lot. If I were to have an extended hedge trimmer, pole pruner, and power sweeper, then I might buy a split shaft unit for those three.
Is your fs85 a solid shaft or does it have a split in the middle? Either way, if you don't use it as a trimmer very often, I would just put the edger attatchment on it. I would buy the straight shaft if you do decide on that.
Here's a close shot of the head on the edger. You can see the huge debris flap and the no-clog design.