The 590 is not that much better.
Possibly. But, IMHO, the 590 is a bit better in design and offers a bit more...particularly for HD. Frankly, if I were the OP, I would hold out and try to acquire a 590A1. You could probably find such a shotgun for roughly $100 more, give or take. Personally, I prefer the 8+1 w/front bead sight. Keep in mind, the 590A1 is a heavy weapon (given the heavy walled barrel), but it does offer the metal trigger assembly, metal safety lever and of course, the heavy walled barrel. If the 8+1 (20" barrel) isnt your thing, they have an 18.5" variant available (Im thinking 6+1 capacity).
I would recommend going with the "standard" variants here, but based on what Ive seen (along with unfortunate, personal experience...ie, the wasting of funds) the 590A1s are usually equivalent in price to the "standard" models. So, again, my advice would be to get the A1 over any other.
Regardless of which Mossberg you go with, I would suggest replacing the stock mag tube spring and follower. I went with a Wolff "generic" heavy duty spring (which I cut and "shaped" myself), along with a Vang Comp stainless steel follower.
BTW- That price isnt too bad for a Mariner. But, I personally question the ability of that particular finish (nickel?) to withstand certain types of solvents/cleaners/degreasers. Again, unless you just crave the look of a Mariner, Id opt for an A1 and avoid the Mariner.