FOr performance, and my only experiences have been with a Ninja 250 and an old '69 suzuki T250, the twin cylinder totally blows away the single.
I am getting much better fuel economy on the GZ than I ever did on my Ninja. (55 vs. 66+)
Single cylinder maintainance is much easier though. You never have to carb synch on a single. Also, valve adjustments and all that other crap are much easier on a single.
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