1) Running costs - repairs, parts prices etc
skyline is heaps cheaper due to more space in engine space, more line on the road also means easy to get parts
2) Performance - which is quicker, handles better
stock 300zx is faster, skyline handle better
3) Mods - is one more expensive to do up than the other
skyline will be more respone to the mod, 300zx get over heat specially after mod due to the engine bay design, not really good for heavy mod
4) Price - what should I pay for an un-modififed version of each.
better check trading post or the for sale section
good luck