I think that an assault/defender SR would last in a grid longer than an SR that's just assault+ (as at skill lvl 20 that's +10% assault/HP versus just +13% assault), but it also depends on the rest of the grid and how much you need more HP. Generally speaking, in a grid that has fewer assault/defender (or pride/defender) SSRs to work with in the first place, the more appealing an assault/defender SR can be. For example, my wind grid still runs 2 separate max copies of Sitry's gun.