To be honest '65' it relly depends on your prefered riding style I think...
ie If you do more long distance riding then the GPZ with it's lounge chair seat will have your posterior,shoulders and neck thanking you every time over a modern super sport.. If you love attacking corners and want a bike that will accelerate at an insane rate of knots , the 10 is the answer.
The modern bikes feel much more nimble given lighter weight , larger tyres , better frame/ suspension..(GPZ feels mammoth after the 10!!) and if things got nasty I'd much rather be on a newer one as they are far more forgiving if you have to ie change lines etc mid corner!!
The new zx14's look... well I reckon they look gay as christmas but maybe that's just me?? The 12's are a really nice sports cruiser..Probably more of a modern equiv' to the GPZ than a ZX10R supersport..
Engine design does not appear to have changed much between the GPZ9's and the modern bikes more they are somewhat more refined , efi etc...
As most will know it's hard to knock the GPZ donk as it was downright brutal in it's day and a cutting edge sports bike of the era...Even the 750 turbo's in the early 80's could cut 10-11 second quarters (same as a modern ZX10R!!).
I am doing a full rebuild on the GPZ including suspension overhaul , new brakes (completely stuffed!) including rotors,calipers and braided lines , all bearings etc..
The stock airbox is proving to be a pain in the proverbial particularly when trying to sort 22year old carbs !!! So I'm looking at other options ATM that will retain a ram tube , be weather/water proof and easier to service......Hmmmm????? (thinking,thinking...)
Take care over X-mas all