I have an SDB and use it to load .38/.357, .44 Mag, and (mostly) .45 ACP. I believe for straight-walled pistol cases, if you don't change loads and cartridges frequently, the SDB is superior to the 550 because it indexes automatically. The 550 is more versatile and will do rifle, but if I were buying a press today, I'd skip the 550 and go right to the 650. (I bought my SDB before the 650 was introduced.)
I'm not Speedy Gonzalez when it comes to reloading, but the SDB will COMFORTABLY let me load a box of 50 in about six minutes. The machine will cycle faster - quite a bit faster, actually - but I like to think about what I'm doing and keep an eye on things. In my hands, this is better than I'd do on a 550. (A buddy has one, and yes, I've tried it.)