I vote 21, the swingarm looks cooler and Tyga do more bits that'll go on the 21 than an 18. I gave £750 for mine as a good runner but with a twisted top yoke and damage repaired fairings. It was generally dirty and neglected looking but in function it was good, which was the important part for me.
Tidy MC28s go for thousands on the other hand though so it all depends on the spec and condition.
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After years of moaning about immigrants now i am one...