If you're on a budget why not try updating the Speedpassions software to the latest version with the variable timing. This is not the same as using a higher fixed timing or using higher timing on the motor.
You can then try your 13.5 and see how you go and then still get a 10.5 if you find it too slow. You will have to adjust your gearing if you use the variable timing on the speedo.
Both of your speedos will be fine with whatever you choose to run but for 13.5 or 10.5 I'd go with the Speedpassion and the new sortware!
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