Recoil of the 30-06 in an ADL should be about like a standard 2.75" 1oz slug in your 870.
I've shot my Mossberg 500 off a bench. It's a little lighter than a 870 and it recoiled like the 30-06 I had. 3" slugs are a different story...I got my first taste of scope bite with one of those and I've never had that problem with the 30-06.
As far as scopes, Leupolds are built very well.
I used to hate them just because everyone liked them (same reason I got a Tikka T3 instead of a Rem 700
).
However, after buying one of the lower end ones, a 2004 model VXI 3-9x40 (has multicoated air/glass surfaces and single coated interior surfaces unlike the old Vari-xI) I decided that the quality was worth the lack of features that other scopes in the price range have.
I did have to send mine back to Leupold though. The adjustment ring was way too tight. They've been very responsive with their customer service.
I wouldn't say you should get a Leupold over any other quality scope, but if you did get one you wouldn't be wasting your money.
Other good brands in that range are Weaver, Bushnell's Elite 4200, some of the Burris's, etc.
What are you using the rifle for?
If it's hunting deer then you should consider getting a small scope from a very good manufacturer over a larger scope of lesser quality or jumping up a level in the same brand and getting the smaller one.
In the field you will probably never even find yourself turning a 3-9x all the way up to 9x anyway (at least around where I hunt).