So, you are thinking about trading a perfectly good 1911 for a "new & improved" Striker fired piece of plastic??
No, in my opinion this is not a good trade. Unless that Colt you got just flat doesn't run, and/or you got one that was made on Friday afternoon after the company "all inclusive" lunch - I would NEVER consider trading a 1911 away...
I own both guns and would NEVER trade my STI for a Sigma unless about a grand was coming with the Sigma. This would be like trading a Corvette for a Cavalier. Both will get you from point A to point B but ....REALLY!
I just checked on GunsAmerica and STI custom 1911's are selling in the low for $927.00 and on the high side for $2,500 Now not seeing or knowing what STI you have I would venture to still say that you have the better gun hands down over the Sigma.
I can purchase used Sigma's for around $350.00.... the guy wanting to trade you knows he is short changing you drasticly, walk away from this while you can.
In my circle you would lose your man card and yoru dignigty. Go to Borders and look for the blue book of gun values. Look up your STI then look go look up that smith. You don't even have to buy the book just look in it.