Eldo 78 or 70, which is better?

It is a matter of personal choice. Both cars have a different look, different appeal. The 78 is a more modern, luxurious version of the large Eldorado FWD. Both the 70 and 78 are end of generation years. A 70 Eldorado is considered far more collectible as a general rule, especially if equipped with rare options: Sun Roof, Track Master, etc. The only real collectible 1978 is the limited edition Tu-tone Custom Biarritz WITH a Sun Roof or Astro Roof to make it even more collectible with LOW production numbers. If you're looking to win a race the 70 is your choice of course. If you were looking at a 70 & 78 in the SAME condition, and same price, your probably better off investment wise to buy the 1970, but this is my opinion based upon the US market of the two respectively.

It will be a nightmare finding parts over there for Cadillac as Cadillac has stopped producing parts for the earlier models. Many Germans will tell you the same thing. Buy a vintage Mercedes instead.