I read every word on the Dillon presses because I do not believe in the approach, "Get what you can afford, upgrade later". I load for several hand guns and a few rifles so changing from one to another was important to me.
I also loaded with both a 550 B and a 650 prior to placing my order. I consider myself a shooting nut, but by some standards am simply a beginner, by others I am more advanced.
The end result is that I bought a 550 B and have never, not for one second regretted the machine I chose. Yea, I catch a little flack from some of the guys at the steel shoots in the area (man on man format) but that has been short lived. I progressed from beginner to master in 12 months, thanks to the 550 B and a buddy letting me shoot his extra gun until mine was built.
The point is, a 550 B was a perfect machine for me. I wanted to load 40 S&W, 38 Super, 45 ACP, 44 Mag, 357, 9mm (for my dad), and a bunch of other stuff. Had I bought a 650, the money would have limited the options due to cost but I could have made it work, and nearly did. Since I bought my 550 B I have loaded ammo all over the place. I enjoy going out the the loader and putting out 500-1000 rounds at a time and the ease and speed of changing to a different cartrige. On a 650 you can do it, and yes, it loads faster, but it takes A LOT longer to change from 40 to 45 and with the time I save in the change over, I load a few extra rounds.
Unless you are going to load for only one or two guns, I recommend the 550 B. It is simple to run, cost effective, and loads fast enough to put yourself into serious bullets in a hurry. You quickly will find you are picking up a bullet, reaching for a case and indexing without thinking about it. To date I have loaded over 15,000 rounds and have never had a misfire or mis feed (I gage my ammo).
No, I am not a Brian Enos (I have dreams of one day being that good), but I am no new B who knows nothing about reloading either. I shoot several thousands of rounds a year (now that I have the perfect machine) and for a vast majority of the shooters, even the serious ones, the 550 B is a good choice.
My theory was to get what I wanted, and I did. I was fortunate in that I had access to both machines and was able to use them both before I bought. I was also fortunate in that one of the guys running a 650, the one I used, was honest enough to say, "I wish I had gotten a 550 but I'm just too far into this one to switch". Yea I still take a little flack from a few of the guys at the range, but I give it right back to them on their mis-feeds and it's all in fun.
I can not recommend the 550 enough. Next will be a shotgun machine, but that's another story. Good shooting to all.