Which has better cooling capability?

I guess that depends on the size of the radiators.

If the aluminum radiator is the stock single core, then the dual core will cool lots better.

i have a kinda on-topic Question: How do you fit an intercooler and a radiator in a z31?

Depends on the intercooler you choose. Some will require you to remove the AC condnesor (and thus loose air conditioning). Others will even require you to move the radiator. But the few kits (HKS and SPARCO) both allow you to retain everything. They mount in front of the AC condensor. But I believe you will have to remove the electric fan first.