Looking at the price of both guns new (to get an idea of where new-gun prices are for these two models right now), the GP-100 is worth a bit more ($50-$60), but it has different grips (Hogue fingergroove) than the current model; the model with the older grips would probably be worth a bit less in the same condition.
When two private/non-dealer parties are trading, then if both parties are happy, then the actual values don't matter that much. If you try to trade with a store/gunshop to get the stainless GP-100 later on, they are almost certainly going to low-ball you on your trade-in, and make you trade against the new gun's full retail price. Under those conditions, yeah, you're going to end up kicking in a LOT of cash; don't be too surprised if your Single Six trade-in doesn't get you to the halfway mark on a new stainless GP-100. It's almost always a better deal to trade with the end-user vs. a middle-man store/shop who will need to mark-up the used gun he takes in on trade for resale.
If it was me, I'd probably do the trade with your friend, and if it turned out that I liked the GP-100, then I'd save my pennies and send it off to a custom re-finisher to get a stainless-looking after-market finish added; something like Robar's NP3 or NP3+ would be a good choice.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)