It all depends on condition and has it been refinished and by who. Most Sistema's were refinished by the arsenals in Argentina. The Glock is a $500 gun NIB $400 or so LNIB so I think you are ahead if you trade straight up for the Colt.
The number one thing to look for on used 1911s is signs of tinkering. Look for replacement parts etc....
-Fast is fine, but accurate is final. The trick is learning to take your time when you're in a hurry." -Wyatt Earp
-Its a tool box... I don't care you put the tools in for the job that's all... -Sam from Ronin
-It is the mark of an educated mind to be able to entertain a thought without accepting it. -Aristotle
-Without deviation from the norm, progress is not possible.-Frank Zappa