Im about to go buy a shotgun to slug hunt deer with im gunna get the 870 express but dont know wich one would be better (accuracy basically) IF theres no difference im most likely going to get the 20ga.

They are both fine. Try different ammo to find what the gun "likes best". I'd go with the 20 tho if it were my choice. Good luck.

With the advancements made recently with slug ammo, It don't matter between the 2. It boils down to personal choice.

My 870 12 beats me up with the slugs, so I got an 870 20ga. for slugs!

Does it shoot good groups? (60 yards)

If you like Remington 870's (like I do) and want a dedicated slug gun, I would look into the new 870 sps super slug. It comes with a heavy barrel for better accuracy and less recoil. I have a 870 express with 23" rifled barrel and cantilever scope mount that wears a Nikon Prostaff 2-7 x 32mm scope that is not a bad set-up, but does have a pretty good kick. A 20 gauge is actually more than enough for deer.

http://www.remington.com/products/firearms/shotguns/model_870/model_870_sps_superslug.asp

I agree with the posters above. I think shooting 12 gauge slugs is about the least fun you can have shooting a gun haha. I cut my teeth hunting with an old Remington 870 shooting 3 inch magnum Remington Copper solids and they were absolutely BRUTAL! If I were buying a new slug gun I would def go with a 20 gauge. The ammo is now just as good as what you can get for a 12 gauge and with the reduced recoil you will probably shoot it more and better.

the 12g on average depending on ammunition usually has a slight edge in ballistics, and a fair amount more impact energy. but thats not to say the 20 doesnt have plenty, as it does. and it does so with A LOT less recoil. 12g. ammo is usually more readily availiable and has a larger selection

x's 2

I don't enjoy recoil any more than anybody else, but I'd go with the 12 Ga. I shoot my slug gun maybe 5 shots a year. I normaly just check the scope with a couple shots, then shoot a deer or two. I really don't notice recoil when shooting at game, so it's not a factor.

I think a pump gun is more versatile in 12 ga, but if this is to be a dedicated slug gun maybe that doesn't matter to you.