I own both the Benelli SuperNova tactical and a Mossberg 590 (which I "converted" into an A1) and too be honest, while I do believe the "better" gun is the Benelli, I tend to stick with the Mossberg as my primary SG (for home defense anyway) given it greater capacity and relatively abundant supply of available parts.
But, having said this, the Benelli SNT is a modern looking, fine shooter and sports many high quality features for the price, such as a chrome lined bore and very nice GR sights (if thats your thing). The chrome lined bore is of particular note as, heck, how often do you see this on a sub-$500 pump? Its also very smooth and the internals are top-notch as well. In addition, Benelli implements a set of screws (one on each side of the forend/pump) that allow you to adjust the action. Ahh...attention to details!
The only real gripe(s) I have is the weak capacity (for the tactical models anyway) and the ridiculous prices associated with the barrels for these guns. Plus, I have no idea why these guns are not offered with a front bead sight only.
So, to sum up...while the Benelli SuperNova is, IMHO, a higher quality shotgun than almost anything out there (for the price) and sports features/attention to details not often found within this category (again...for the initial cost), it is not the most practical. Thus, this is where the 590 steps in.
Of course, all of this is simply my opinion. Best thing for you to do is go out and handle a few of these weapons. I doubt you will need to actually fire one of them to know which one "fits" and "feels" best to you. Give them a try. Either way, you will end up with a fine shotgun.
As a side note, if you are looking at the 590, might I suggest paying just a bit more for the 590-A1 as it offers many features such as a heavy walled barrel, metal trigger assembly and metal safety. These guns are really not much more than the "standard" 590. I learned this the hard and expensive way!