I have both an XL650 and an RL550b. I have had a SDB previously.
If I was only doing one pistol caliber and only a few hundred rounds a month, I would say go with the SDB.
More calipers and/or rounds per month, go with the 550.
I do .45acp on the 650, 9mm and .223 on the 550, soon to add .300blk, 10mm and .308