(Yes, I'm serious about getting one - http://www.fiatspider.com/f08/viewtopic.php?f=3&t=17115)
Keep the 1980 spider, or keep the 1975 spider? Which would you choose, and why?
1980

Good:
Automatic (wife will drive it too). Driving condition, except for the occasional hiccup. We're the 2nd owner and have a decade of documentation. Fuel injected. A few $K of improvements on our part (brakes, soft top, etc). Recent tuneup.
Bad:
Rust in a few spots like the corners of the doors, mostly not structural. The one bad spot I've found was two weeks ago - the passenger door sill is eaten through (just the horizontal bit, not the vertical supporting members...or so it looks). Mediocre/poor repaint job. Automatic (boring!). Hardtop is slightly damaged (I've been intending to sell that anyways). Rain just pours right into the doors, so the cause of the sill problem is obvious. The car is not running right now due to electrical issues.
1975

Good:
Original factory paint is still bright, just a few scratches and one or two dents. Almost zero rust. Hardtop is in good condition. 5-speed, shifted great when driven last. Pile of big spare parts collected from Craigslist (windshield, trunk lids, etc).
Bad:
No documentation. Transmission leaks, but have spare. Coolant leak, but appears easy to resolve. Most systems need some TLC (brakes, timing belt, turn signals don't work...). Likely needs a tuneup. Need to replace the engine crossmember. Needs new soft top. All of which takes time. It's been sitting for two+ years because we've been working on the house, toddlers, and the 1980 Fiat.