Depends on what you need a progressive for. Both are great presses for what they are designed for. Quoting Dillon:
If you want to leave your dies preset, you'll need:
Toolhead(#13863) and a powder die(#20064) OR The 650 Quick Change(#22059),which includes a powder measure, powder die, toolhead and a toolhead stand. This allows you to leave your dies and powder charge set up and dedicated to a specific cartridge for faster, easier caliber changes.
So at least you need a powder die in addition to a tool head. Add to that a shell plate if the one you have isn't the right size for the new caliber. The Quick Change kit has a whole powder measure. That's where it gets even more expensive....and extra primer tubes. If you buy a Case Feeder Collator, then you have to by caliber specific parts for that as well.
The RCBS Pro 2000 is made to make caliber change simple. You still have to buy die plates (tool heads in Dillon-ese) and a shell plate if case base is different, but the powder measure is simpler. Adjust the mic for different charges...so you only need one P.M. Simple means no case feeder and negligible primer system change effort. (10 seconds tops). Everything on the Pro 2000 is simple. Fewer moving parts and no post setup syncing and calibration. Once is enough. In a nut shell the Pro2k is cheaper to own over time than the 650. If you choose the Pro 2000, you load a lot of pistol,
and you buy a bullet feeder, then it's worth while to buy extra powder die/brackets to keep on your die plates and buy powder-through case expanders to drop in. With those two parts, you can make the Uniflow expand and charge in station 2 like the Dillon does. Then it becomes just as fast as a Dillon, plus you can put a lockout die in the stationary #3.
Without a case feeder the 650 is inconvenient....cases feed from the handle side, bullets feed on the opposite side....
so if you get a 650, plan to get the collator.
The Pro 2000 is plenty fast for all but competition loaders feeding both cases and bullets manually from the left side. But you can speed it up with a bullet feeder. Spend $30 for the simple Tube Bullet Feeder, which can be upgraded to the collator-fed one (for a price). I use a Hornady bullet feeder modified with clear tubing and a stop switch. That not only speeded things up, it simplified things. A bullet feeder doesn't increase caliber change time more than a minute.
The big Pro 2000 advantage is the APS primer system. Much faster and safer. The "faster" comes when you buy CCI primers preloaded in APS strips. Otherwise loading a 100 in four strips with the Strip Loader is about the same speed as loading a 100 in a tube.....just safer.
Yes APS is a love/hate thing. Hate if you don't have the patience to digest the instruction sheet and learn a new system. Love if you do. Hate if you own anything other than a RCBS Pro 2000, love if you do.
I loaded tubes for many years....not likely to buy RCBS's conversion kit for tube primers. IMO that would be really dumb.
The Pro 2000 is simple enough to even make a trouble-free quick changing case feeder. I did. Cost less than $50.