Vtnick - I will try to help you out as best I can with your decision. Keep in mind not to take what you read as gospel but only one's person interpretation.
First and formost these shotguns are mid price level mass produced semi-autos and to compare either one to the several high end ones would be a mistake. Cause when you pay in excess of thousands of dollars for an high end shotgun you expect it to be flawless.
The 930 spx and 459 tr will give you well way more bang for the buck, but thats not to say neither of them have flawless points.
Mossberg 930 spx in 2009 thru 2010 was all the rage and demand for them was outrageous. In their haste to mass produce this model I believe quality control went out the window at the mossberg factory and a whole lot issues started to pop up when it shipped out to customers. Such as canted front sights, loose rear sights, missaligned elevator, bent mag tube and/or extensions, chambering misteps and host of others problems.
The piston action of the 930 spx is fairly simple in design compared to the bennelli's inertia system. But on close inspection of the internal workings I see alot of rough surfaces and sharp edges where metal meets metal. This again should have been taken care of at the factory.
Later produced model seems to be doing a lot better and some of the problems were corrected, current production models will have a larger charging bolt. There is also a Blackwater 930 spx edition that kind of remedied some these factory flaws. In my opinion once all the flaws were reduced the 930 spx is the most reliable tool to have in your arsenal.
Weatherby's 459 TR (tactical response) is their first foray into the Home defense/Tactical catagory. Little is known about this 12 gage but upon closer inspection everything seems to be well designed and put together right from the factory. The piston action is different in its design but similar like mossbergs in function, very smooth with no discernable tool marks or rough surfaces. Pistol grip stock, forend, rear and front sights is very clean in the fitting department. Added bonuses a choke system to get you a few more yards, two piston bushings one for light loads and one for heavy. Other than the 4+1 capacity the 459 tr seems to be the epitome of a home defense weapon. Downsides to the 459 tr is that its relatively new and aftermarket parts are very hard to find nationally. Kind of a quirky chambering of a round, normally when you rack back a charging bolt a shell is release on to elevator and into the chamber. No so with the 459 you have to depress a carrier lock button on the left side to feed a round and then a rack to the back to chamber. Weatherby's factory is overseas and warranty work may take additional amount of time to resolve if ever a problem arises.
TANGIBLES:
The 930 spx has some heft to its feel-equally adequate out at a camp site or the house. Strange as this may sound but that extra weight out in the field gives me a little more sense of security.
The 459 TR for all intense and purposes is light and tight. Even though both guns are equivalently the same, the way the 459 tr is packaged gives it that easily manipilated feel to it. Even the Wifey will be able to post up and do some damage if thats a consideration. To me if its ever needed having that light and tight control in close quarters could mean all the difference in that fraction of a second in shot placement.
INTANGIBLES:
Mossberg 930 spx - Why the numerous flaws in what could be an outstanding firearm.......why mossberg??..........why? Also the pistol grip from choate could have been done way better or offer the original straight comb back to its customers.
Weatherby 459 Tr - Very scarce aftermarket parts, your kind of stuck with the way its dress, unless you are a talented gunsmith. Limited loading capacity unless you do a extension mod and it may or may not void your warranty. Due to the factory being overseas and its unknown if Weatherby will have a stateside service center for warranty repairs.
OVERALL: Slight edge goes to Mossberg due to availability of aftermarket parts and the factory is in the USA. Hope this information gives you a little more insight to your upcoming purchase.