The difference between a better reg is durabality, consistency and flow; keep in mind expensive doesn't 'always' mean better.Durablity = self explainitory
Consistency = how well the reg maintains a fixed pressure from shot to shot, or day to day.
Flow = Flow is the regs ability to move air through it at a fast rate. This is also referred to as "recharge rate." If a reg has bad flow it will take longer to recharge to the set pressure, which means if you fire fast your second shot will be a lower velocity than your first (shootdown)--worse yet your blowback marker may sputter on the second shot because the reg wasn't given time to recharge to the pressure needed to recock the marker.
The shutter reg is known to break so it's durablity is questionable, it also has poor flow especially once the pressures drop below 600psi.
There's nothing wrong with using two regs; two is better than one in most cases. It's a must that you leave at least a 150-200psi difference between the two regs, so unless you have a low pressure spyder it will likely be fairly hard to use to regs correctly.
If you want to keep you shutter reg installed instead of buying a new drop forward, you should open it up all the way so it's basically doing nothing. This will ensure that you will be getting as high of flow through the reg as possible. The gauge will remain useful as a way of seeing how much air is left in the tank. I used a palmer reg with a kingman reg on a spyder 2000 and I found this to be the most reliable and consistant method.
Home