I think the Tar-Hunt company will customize Rem 870s to make them accurate, dedicated slug guns. They pin the barrel and do some other modifications for around 500. If you already have an 870 this may be the way to go.

If I were to buy a slug gun right now, it would be the Savage. I might buy the 20 over the 12, but I'd have to check the latest ballistics.

But truthfully, my Mossberg 500 with smoothbore slug barrel - open sights and rifled slugs (at less than $5 per box) would easily kill every deer I've ever taken with a slug except one.