A few factors to consider when choosing a prop:

1) Max static thrust
2) Spool up response
3) Air Velocity VS Static Thrust for "hovering" RPM

1) For the same peak RPM, usually a lower pitch, larger diameter will yield greater static thrust than a higher pitch prop. This is only true to an extent. Because when you go to too low of a pitch, the tips have next to zero angle of attack, which generage next to zero lift, although RPM is impressive. Also, max. static thrust is a priority only if your power is marginal (1.2 ~ 1.4 thrust-to-weight ratio). If you are already at 2:1 thrust-to-weight ratio, you should use the power in other aspects, like speed.

2) If you learn to fly w/o having to gun throttle constantly, maybe you don't need fast spool up. Also, higher nitro (in 4-strokes) help spool up.

3) This is what your "advanced 3D'ers" may be referring to. Consider a plane at hovering throttle, at which the prop @ XXX rpm is generating the static thrust equal to the weight of the plane. A higher pitch prop will yield a higher velicity air flow in a smaller disk, as compared to a lower pitch prop. This will make a difference in tail control response. So for example, if you massively over-power a plane and use a low-pitch prop (for your case if you use YS 120 for a 7.5 Yak profile, swinging APC 17x6), the plane will hover at just above idle, maybe 3000~3500 rpm. This is enough thrust to counter weight, but I doubt it has enough airflow past the tail for good tail response. I have a 5-lb Funtana w/ Saito 91. When I use 15x4W prop, it hovers @ 1/4 throttle, tail response is simply not there. When I switch to 14x6, it hovers @ more like 1/3 throttle, and tail resonse is a little better. Of course, if my Funtana is 6 lb, or if the rudder is larger, it's a different story. But it proves the point. A higher pitch may not be a bad thing for Fun Fly, especially if you already well surpassed the min. thrust-to-weight ratio.