The 18 was (for the most part) not restricted, so made decent power from the word go. However it had 18 inch rear wheel, which means slightly less choice of rubber. The 17inch MC21 wheel will go straight in though.
On the track there's really nothing between them.
Looks are personal, and I think the 18 looks dated whereas the 21 doesn't.
Tuning parts are readily available for both, and many parts will interchange as far as engines are concerned. You can even go as far as fitting the entire wiring loom and gubbins from the 21 if you want to.
Bolt on goodies include HRC jet kits, ignition units, reeds, stuffers, and all manner of pipes / cans from various manufacteres. Tyga are popular, because the guys know and love NSR's, and are English, though they work from Thailand.
How deep are your pockets? How much power do you want?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
