I'm new to reloading, I have a Square Deal B.
The FAQ has a great breakdown on the difference
http://www.brianenos.com/pages/dillon.html
Pulled from the above link:
Should I get the 650 or the 1050? A common question: I could probably afford the 1050, but do I really need it? I asked this question to three friends and long-time reloaders at Dillon Precision. Their answer: If you'll only load one to three-thousand rounds per month, and plan to switch calibers from now and then to frequently - get the 650. If you plan to shoot 50,000 to 60,000 rounds a year (4,000+ rounds/month), especially if don't plan to switch calibers - get the 1050. So, if caliber changes will be involved, lean toward the 650. If you don't plan to switch calibers, and would just like to set down at the machine and crank out some serious ammo when you have a few minutes - get the 1050. One usually knows when they're ready for the 1050. If you're not sure, get the 650.
They do not recommend the 1050 for a newbie at reloading, & it is a commercial machine hence the different warranty.
I do not shoot nearly enough to justify the 1050. YMMV...