I came from an S14a and in the same usage I'm getting about 5 mpg less.
The Z has less faults and rust isn't an issue but when bills hit they're generally higher (Tyres £600+, Brakes £500+, Clutch/Flywheel £800+, Exhaust £500+).
Depreciation is high, although no worse than your average family hatchback (~£2k per year). I had my SX 4 years and only lost £1,500 on it! Prices should stabilise in a couple of years because a clean UK 350Z will always be worth £5k+