You'd definitely find a lot of good advice over on the C&V forum, I'd ask there as well.

The Schwinn definitely looks like it is the 1986 Prelude. Here's a link to the catalogue if you're interested : Schwinn 1986 Lightweight Bicycles -- Prelude. If that's the case, the frame is made of Columbus Tenax tubing and it'd be equipped with Suntour Cyclone derailleurs. It's a pretty nice bike in my opinion and I've heard a lot of good things about the late 80's Japanese made Schwinns from various forum members.

That being said, I don't think you could go wrong with the Diamondback either as per the reasons above.