The actions and operation are all the same between the three models you listed. The SPX is a shorter 18" barreled "tactical" shotgun with a 7+1 shell capacity designed with law enforcement in mind. The Blackwater edition is an SPX with a few different bells and whistles, Blackwater logo, slightly different sights, stock, etc. It's the same shotgun in a different dress. These models have been better in recent history, but have been plagued with function and quality control issues from the start.
The basic 930 is set up for field use. These have longer field barrels, basic bead sights, a 4+1 shell capacity, and have a reputation for being far more reliable than the tactical models. There is a 930 Field/Security Combo out there that in my opinion offers the most bang for the buck...
http://www.budsgunshop.com/catalog/product_info.php/products_id/76743/Mossberg 930C 12 18CB28VR ACCM SYN
This model comes with a 28" vent rib barrel with their Accu-set chokes as well as an 18" security barrel for home defense. It's a great deal for all that comes in the package and covers the span of what one might need a shotgun for...