I have tried to find this specific information and I can't seem to find an accurate opinion.

If height adjustability isn't an issue (in order words, the 350Z H-Tech drop was good enough), what would be the better performing setup?

BC Racing BR Coilovers

or

350Z H-Tech Springs + Tokico D-Spec Shocks?

They are both dampening adjustable. It seems like some say that the D-Spec shocks are better quality, but most users seem to swear by their BCs.

Advantages? Disavantages?

 

From what I have read the tokico's will give you better overall handling supposedly... but then again you are matching it with springs it wasn't necessarily designed for where bc's should be better matched. But unless you are going for pure track I would go with the coils so you can choose your exact height and don't ever have to worry about changing springs or having one side higher than the other, etc.