The RS125 is a focused trackbike, with a frame, suspension, and even engine to match. The ninjette is a streetbike that happens to be fun to take to the track due to its lively handling and relatively light weight. There really shouldn't be any comparison in terms of "which is better" on the track; it's not even close to a fair contest. As you mention, it's unlikely they'll sell more than a few hundred in the US anyway, there just isn't that large a market for a track-only 125. Though those that do get into it are undoubtedly quite happy with their machine.
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