What are the merits from a front multi-link design (300ZX/Q45/Skyline) versus a double wishbone/A-Arm design (FD RX7, Miata, etc.)?

the double wishbone has minimal suspension geometry changes under load so it performs better on track, which is why miatas, rx-7's etc are all more popular than almost any nissan at track events. They handle better.

i may be wrong on the actual reason, where are the suspension geeks?

Double Wishbone Suspensions are simpler and easier to tune. The offer the advantage of giving negative camber when the suspension extends and positive when it compresses, thus working to keep the tires parallel to the ground. On the other hand, a Multi-Link is a VERY tunable suspension. if you know what you're doing than the multi-link can adjust one parameter (scrub radius, motion ratio, camber curves, etc.) individually, while on a wishbone, you have to adjust a couple different things to get one thing changed and keep the rest of the parameters the same. Overall they are both very good suspensions that will out perform McPherson Strut suspensions and the like.

Keeping this in mind, remember that most (all?) new Porsches utilize McPherson Strut front suspension.

My 68' mustang had a double wishbone style front suspension. It handled like crap because it was poorly engineered. (It actually gained positive camber during compression...still miss that car though)

Final point is that with the right parts and tuning you eliminate or at least minimize any downsides to your particular suspension.

of all the cars ive alligned the dbl wb cars had more adjustability from a stock parts standpoint. u could set the caster toe and camber wherever u want within its range if u thought it helped.