The way i see it is that yamaha have taken the R an thought "how can we make this a better bike" and they came out with the Z the more versatile of the two straight out the box.
The main reasons i bought the Z and Not The R, are:
Looks
Tank range
Huge road presence!
Just as capable off road
more comfortable
So on and so forth...

Although if the R had the bulkier swingarm and Twin front disk i would of seriously considered it.

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...On a steel horse i ride.