Unless you're a lefty, I don't see the speedfeed stock being a problem, and mine has no shell retention problems even with 3" loads. I actually like the speedfeed stock better than a sidesaddle, it's one less doohickey sticking out to get caught on things and get broke (since most are plastic anyways).
And keep in mind-the 590SPX doesn't come with a heavy barrel, metal trigger group and metal safety button. But either way, you'll be satisfied with either. Also, you'll have the bayonet off the gun about 99.99% of the time
*EDIT* and for the price of the SPX, geez, just buy a $20 rail and put it on yourself; all mossbergs are drilled and tapped already. If you absolutely have to have it, it's a two minute job. It even comes with the little allen wrench