Stating the obvious, the aesthetics are a matter of personal opinion. I much prefer the Mk1, but the majority seem to prefer the facelift.
Drive wise, I doubt there's a noticeable difference from the very minor suspension tweaks to the later model.
Engine - identical.
Exhaust - identical (too quiet).
Interior - slight changes to some of the plastic colours on the Mk2 and much nicer seats.
Parts for the Mk1 seem to be more expensive e.g. Nissan quote for rear bumper - Mk1 - £1,200 + VAT. Mk2 - £369 + VAT.
Price wise the older model seems to be holding its value fairly well so you won't save a fortune by going for it over the Mk2.
Both are rare on the road. I've never seen another Nismo. The Mk1 will be rarer due to its short production run.
For all its deficiencies in the eyes of some and negative comparisons to German rivals I'd absolutely recommend one - either version. Puts a huge smile on my face every time I look at mine and every time I drive it.
Hope that helps.