Disco 1: Simpler, easier to maintain, cheaper to buy.
Disco 1: More luxurious, safer, quieter, wider, twice the rear storage capacity, more horsepower. Traction control works great, when it's working. ABS works better than D1, when it's working. The rear overhang is a PITA for serious off-roading, but it can be improved a lot. Harder to repair due to more proprietary "blackbox" systems and modern vehicle design (unit bearing hubs, etc.) But, will have lower miles, better condition, etc. on average, just due to the age difference.
That's it, in a nutshell.