300zx NA with axles 4:O8 gears- bolts into car no driveshaft shortening
Pre 95 J30 with axles 3:92 gears- bolts into car but needs 1/2 off driveshaft or ABS driveshaft(measured drivesafts today)
*note some people have made it work by compressing their center support bushings( not ideal)
95+ J30 3:92 gears-bolts in but is the same issue with driveshaft that Pre 95 has.
I am listing all this just to give you all the info.
which is better? well they are all R200 VLSD's the only differences are gearing .
what are you planning on doing with your car?
If you are going Turbo then the J30 would be better suited for you due to the 4% difference in gears . Thereby keeping your engine in boost a little longer in each gear.
remember in Japan and Europe S13 SR20DET's S14 SR0DET's and S15 SR20DET's all have 3:70 gears because of the Turbo setups just like the gears in a 300zxTT which are 3:70.
Now if you go NA then you could run the J30 some people do and claim not to be able to notice the 4% difference.
Though if you want to drag race then you would probably want to stick with the 4:O8 gears which means a 240sx VLSD or 300zx NA .
Well thats everything you have to figure out what works for you .
Technically no one rear end is better it just depends on the application.
hope this helps
edit: it was doing <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/wow.gif" border="0" valign="absmiddle" alt=':0'> instead of zero, just trying to help