Re: progressive press for 45 auto
I say skip the 550. A solid machine, but the lack of auto-index is like getting to have sex with a super model, but not being allowed to get off.
If you are absolutely, positively...beyond a doubt only going to use the press for loading pistol, the SDb is a great choice. Inexpensive, compact and auto indexing. Leave it all set up, walk up to it and churn out some ammo.
The Hornady is a great machine in it's own right - and your top pick if you think you'll want to do rifle as well - which I highly recommend.
I have a 550, 650 and the LnL, and between the three, the 650 and LnL are close, with the nod going to the LnL, and the 550 coming in a DISTANT 3rd. I hate, hate, hate the lack of autoindex, spent primers on the floor, and lack of 5th station (for a "versatile" machine). I give the SDb a pass on its lack of 5 stations, because it is a dedicated pistol rig.
How much ammo are you looking to make?