While some people like to tinker and continually change things, sadly, I've found that when I setup a gun for a particular ammunition and it's shooting well, I tend to leave it alone. My Impact is setup for the King Heavies (.25) I can shoot the standard .25 pellets, but it's set for the King Heavies. I won't be buying a slug barrel simply because I'd have to set it up again for the slugs. Too much hassle.
To make a long story short, if you plan on shooting both pellets and slugs, get two dreamlines. Set one up for pellets, the other for slugs. It'd be a couple hundred more than and Impact (Impact $2000, two Dreamlites about $2200, plus an extra scope, or two Dreamline Classics for $2000 plus the extra scope), but you can setup one for pellets and one for slugs. Then there's no hassle in switching from slugs to pellets and back.
Now, as the others have said, you can't go wrong with an Impact or Dreamline, but for one Impact you can get Two Dreamlines. After watching the UAG video, the Dreamline ain't no slouch.
I pity you having to make the decision. I'd hate to be in your shoes.