The 9x4 probably won't work. It won't give the plane enough speed.
The 10x6 might work. It will make the motor work harder and will give you a little more power (although possibly a bit less speed), but the battery won't last as long (the motor might not either! [img]i/expressions/face-icon-small-shocked.gif[/img] It depends on what motor and battery you're using).

Find a place that has the right size of prop and buy at least half a dozen of them.

Also, make sure they're electric props, not glow engine ones (which tend to be larger and shaped differently, and also have a larger hole in the center). 10x6 is the most common size for glow engines.