You'll probably have the fewest problems with the new one. A completely brand new bike, and one that has 25 years of technology advancements and changes, has the inherent advantage.

That being said, if you have the ability to re-build the vintage one to your preferred specifications for the same price or cheaper, I'd take the old one. Something magical about the older bikes. You just have to remember that you might have more problems sourcing replacement items, have a frame with more scratches, and it will take more work to get up to speed.