I have both the 550B and the XL650 Dillons.
I have had my 550 from nearly day one of it's introduction. Prior to that I was using a Dillon 450.
My 550 has 7 extra tool heads, each with it's own powder measure, set up for various calibers that I load/shoot. It is setup with the bullet tray, strong mount and roller handle.
The 550 is probably the best all around machine for the average shooter. It will and can, load all the ammo the average shooter will need.
Parts for a change over are cheaper for the 550 than the 650 especially if you have to change the disk in the case feeder.
Changing from small primers to large is a simpler task with the 550.
I bought the 650 this year because my son and I were really burning up the 9mm ammo in GSSF competition (10,000 + rounds). We needed to spend more time on the range and less reloading. This we accomplished.
The 650 with the case feeder, bullet tray, roller handle and strong mount is a very fast machine. It is only faster than the 550 due to the case feeder but, it is faster none the less.
I typically only do 300 rounds a sitting. Is the 650 worth it?
No... but you would be able to do those 300 rounds in about 15 minutes and not break a sweat.
It takes longer to get ready to load with the 650 than it does to actually complete the process.
The rest is up to you.