I have both.
The 88 is great if you're going to be lugging it around/have it more as a portable shotgun, its light and made of aluminium. It's action has more of a small resistance so it does not easily slide open with the smallest pull force (not a bad thing, but comes down to preference). Accepts all Mossberg 500 model accessories (except the foregrip as it is pinned). The bluing finish is easy to wear off, it may help to run a cloth with a dab of lubricant all over to protect it from condensation if you're keeping it outdoors a lot. Mine has been through hell and back, some rust got into the trigger group and barrel exterior (which I was able to clean off) and it still runs as smooth and flawless as when I bought it for $170.
The 870 is heavier, more solid and has no resistance on the action, its an awesome range/bird gun. I'd suggest this one for more stationary shooting. It's got a high quality finish, less parts to assemble, more modifiable magazine tube, and has more accessories available to it. The trigger is excellent and the hammer hits with some real authority. Its a beautiful shotgun, I like to keep my 870 on the wall and the 88 in the truck.
I also have a 590A1 and a Tactical 870, these ones are fun too, nothing overly special/different than their economic counterparts but I think they make much better range toys lol
I'd stay away from Winchester SXPs, the prices are comparable to Mossberg and Remington but I didn't see the quality there when I had one.