If you felt confident enough to buy the 28, then you should be confident enough to do the 3 day test. It includes the CBT anyway doesn't it?
The nsr will probably be a safer ride than the CB100 in any case based purely on the handling, braking etc... If it's standard it's not gonna bite unless you do something ungodly with the throttle anyway.
GO for it girl!
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NSR300R - Why did i ever have a 250...