Gotta say either keep the Sig or make sure you get enough cash in the deal to make replacing it affordable. Around here a used SS GI might go for $350 - $375. You're talking about a very basic 1911, not a streamlined model with all the bells and whistles.
How's the hammer bite situation with extended range sessions? My Mil-Spec's a flat out Man Eater - I've been shooting 1911s for a good many years so I'm willing to accept that, to a degree, on this one particular specimen (future 1911s will have a beavertail).
I understand you want to help out a friend and there may even be a bit of patriotism in your motivation to sell ... but the Sig's STILL a much better, more refined weapon.
Would the Springfield be a safe queen, a range gun, a "tweaker" or a future CCW? That'll affect how much extra money it's going to cost you. It's a good platform to build on for sure but that gets expensive quickly. I brought my used Mil-Spec home for $65 plus my traded in M66, I've now got $800 invested in the ol' girl ('smith work, parts, leather, etc).
Either way it'd be a tough choice ... he's offering $200 extra in the deal? That sounds about right for my area. You could knock that down to $150 and look at it like he's getting a good deal on a solid duty weapon while you're doing right by a friend.
How important is the money to you?
If the SIG is something you normally keep in the safe, how different would that be if it were the 1911?
We all like to get a good deal. That's what we're all looking for, isn't it?
You are in a position to give a good deal. That feeling should count for something.
Money is not everything, however, it does help pay for my gun habit! ;)
I think you're in a good position to do a good thing. Trust in that.