it all about matching the power to the load just like gears on a car.
the simplest calc I've seen is

Diamerter sqarded times pitch. So a 10x6 prop wouls have a load factor of 600 a 10x7 would have aload factor of 700

the reason a 10x6 would be faster is the load factor is less. So it accalerates faster and run a higher rpm than a 10x7 prop. Kinda like when you down shift in a car and punch it you have more power in the lower gear, the rpm gain can overcome the speed differance.

basically you need to take a couple things into consideration when selecting a prop

what sized motor - manufaturer gives you recommended props and recommended rpm ranges. follow them

next decide what you plan on doing wwith the plane. Is it scale? what kind a lumpering cub. ya want a big low pitch prop to just truck around the sky with
for a 46 12 4 or 5. On smaller pattern ships a prop like this is ideal also. It won't go fast but It'll go straight up.

you doing quicky stuff, speed is all your worried about. you want a small high pitched prop. 9x8 or 8.75x10. you'll run like a bat outta hell but your acceleration will suck, may even have problems cavitating the prop at slow speeds. notice the calc on the 9x8 = 649. Split between the 10-6 and 10-7. This is backed up with the RPM with either prop being within a couple hundred rpm, get out a tach and try it.

or is it a sport model you want decent climb decent speed. a 10x6. A 10x7 probaly won't increase the speed if it's a weaker 46. It usually recommended on break in to limit the RPM. You don't wanna exceed red line when breaking in a motor or ever.