The 550 is $406 and the 650 is $529 so the first difference is $123. The 550 can load over 160 calibers, the 650 does less though you should refer to their web site for the exact differences in calibers.
A 550 caliber conversion kit is $55.95 and the 650 is $73.95. The 550 is hand indexing and the 650 is automatic indexing. The 550 is still the most popular and highest selling of the Dillon machines.
The 550 has 4 working stations and the 650 has 5. For most people 4 is plenty but 5 is better. Most people who have a 650 leave one station blank or fill it with the powder checker. Both have the optional case feeder the 550 is $239 and the 650 is $209.
Each press has a following which splits the Dillon crowd similar to the Dillon vs Hornady crowd.
Both are blue and both a very good machines being able to reload rounds faster than a single stage press by leaps and bounds.
They are easy to set up and adjust, each offers interchangeable tool heads for those that do not like to rest their dies. By adding the additional powder funnel for $72 you never have to change settings unless you change powders.
The function of both machines is smooth, one you hand index the other pulling the handle does the indexing. Changing from large to small primers is pretty simple and will take you a minute or two. The directions are clear and they have a great support system, of course replacement parts are free and shipped fast. (other companies offer the same service)
The powder measure works great though some powders are a little harder to adjust. However the powder drop is pretty constant across the board. Modification can be made to the powder measure to help set the charge weight such as putting a better knob on the adjustment bolt. In addition you can polish the metal parts of the measure to improve dropping for the harder powders. Each comes with an audible low primer warning system.
A plus enhancement for both is the roller handle for $40. I would suggest extra small and large primer pick up tubes so you can load them and hardly ever have to slow down to load primers.
A better web sit for more information is
http://www.brianenos.com/store/dillon.html
Dillon's RL 550B has all the simplicity and reliability of a single-stage reloading press, but it loads a complete round with every pull of the handle. And it's as easy to learn to reload on as a single stage!
The 650 was designed with the serious competition shooter in mind – it’s reasonably priced, and it can crank out a boatload of ammo.
Read this link for help on the differences at a fast glance & read.
http:
//www.brianenos.com/pages/dillon.html
I have the 650 and it is a great machine though I would not be that quick to say that the 550 might not be the better machine for you. Please read the second link above and make you own decision. You might note that seasoned loaders do not consider the powder check a make or break issue.