An interesting situation.
I wouldn't pay $1600.
But I'd get the Colnago back. On flat land you're not going to see the benefits of new frames, assuming there are any. I'd also strongly suggest upgrading the running gear.
I've thought this over a good deal, almost buying a retro frame to set up. And considering repainting my 1974 bike and setting it up
Now my "old" OS paramount is coming back and I plan to full DA it and ride it.
I'm riding primarily a new CF Italian fancy bike, Campy etc., and it's great. But it doesn't make much different on the flats. Climbing and descending, yes, but on the flat who cares. So the oldster will do as well.