I own a 550B and an XL650. I am also preparing to get a 1050 on my bench as well.
With that said, I believe the mostly highly appreciate press on my bench, including the multiple single station presses, is my Dillon 550B. The press is simple, basic, quick, and forgiving. If you make an error on an auto-index press, it can be highly frustrating to move shells around, adjust again, then get back in the groove. That simply is not the case with the 550B. It makes you think more instead of just pulling a handle and having ammo pump out. Don't get me wrong, I love my 650 and have loaded many of rounds on it, but I think if I had to have one progressive press (or maybe even one press entirely?!) it would be the 550B.
Caliber changes on any progressive will take a little time, but you get better with experience. People often claim the conversions to cause headaches, take time, and to be a pain. I believe that this is over exaggerated for Dillon presses, and is often mentioned by those who do not actually own a blue press. Does it take a few minutes? Yes. Is it rocket science and terribly difficult? No. That is coming from a perspective of a 550 and 650.
Personally, I wasn't a big fan of the SDB. I couldn't tell you exactly what it was or why I wasn't, but I just didn't dig it.
Since you had asked, I will go ahead and give my honest opinion. Buy the 550B. It is truly enjoyable to load on and is what I like to think of as 'relaxing'.
If you load enough of multiple calipers, buy extra tool heads and powder dies and keep your dies set up in them. It will make the conversion even quicker. If you have money burning a hole in your pocket, buy multiple quick change kits; otherwise, set the powder dies on each toolhead, and use one powder measure for all the toolheads. Just remove it and transition it to the next toolhead and set it up quickly for the charge you want.
Also, skip the unnecessary accessories. In my opinion, a roller handle is an unnecessary addition. Your hand is coming off the press after every stroke anyways, which defeats the purpose of the roller part of the handle. I don't find the ball handle to be uncomfortable, even after sitting and loading 1,000 rounds straight. Also, low powder warning is also not necessary. You have to add primers every 100 rounds anywas, so make it a habit to check the powder as well when doing this. The strong mount is hit or miss, I have had my 550 on one and off of one, and liked it both ways. I see the benefits of it though, that's for sure. Also, I'm not sure I could see the need for a case feeder on the 550. If you're getting to the point you could benefit from one, the 650 is probably the better machine to go with. You can still crank out large quantities of quality ammo on the 550 without a case feeder and without the extra add-ons.
Like I said.. These are my OPINIONS and should only be taken as that. I know others will disagree, but that is the purpose of having individual opinions.
If you have any questions, feel free to PM. I could take pictures, walk you through questions, or even jump on the phone with you to discuss my Blue presses.
They are expensive, no doubt. But! They work, they are reliable, and they are enjoyable.
