If you're gonna give the 300zx the attention it deserves then its a fun route to choose, but if you're just gonna try and do bolt ons and an ECU and try to beat the shit out of it for as long as possible and constantly be playing catch up fixing things as they break then you're going to hate it. When i bought 94 300zx TT a year and a half ago i immediately did a built motor, garret gt35r turbos, 3" downpipes, 740cc injectors, intercoolers, dual intake, full exhaust, and the car runs great and is very reliable, but i've still had to fix things since then like the steering rack and AC compressor, and that doesnt' include the fact that you need to do some good coilovers and bbk in order to make it safely handle the added power. most 300zx owners i know just try to milk them for what they are worth, they never work right and then they just get fed up. Just an FYI i've prolly put about 15-20k into my car since i've bought it, so unless you are willing to spend the time and money required i would say just do the 335.
I also have a 2008 fully loaded 335xi e92 with a cobb AP and i love it also, and i'm very happy with the combination of the 2 cars. But yea, if you're not gonna spend the time and money that the 300zx needs to really make reliable power then i wouldn't bother with it. But yea, its a great combo if you are going to do the two of them.
As far as the 'vs' is concerned, i bring my 300zx to most of the bmw/bear mtn/etc meets and my z32 is faster than any of the FBO/tuned one's i've raced... my z makes 478whp/482tq on 93 octane.