It is kind of like real-estate, only here it is...
mission, mission, mission.
Some people say they want the stronger airframe of the -7 and they don't feel comfortable flying a plane with the "low" limits like the -9 but they continue to fly Cessna's and Pipers with the same ?low? limits while building. Duh!
Other's comment that the -9's fuel capacity isn't enough. I call BS on this because if you install the engine designed for the airplane, there is more than plenty of fuel. (Some say they just pull the black knob back to reduce fuel burn. Ok, if I had 180 HP up front, that black knob would be all the way in, all the time.) The 75% power range of a 160 HP -9 is 710 miles, the 75% range for a 180 HP -7 is 775 miles. I?m not sure that 65 miles is a deal breaker.
Others talk about wanting better climb performance the larger engine provides. Better than what, a C-172? My 135 HP O-290 powered -9 goes up around 1400 FPM at gross on 90 degree days. How much more climb do you need?
Just doing some myth busting here.
As for why I picked a -9 over the -7...
My wife and I hope to use it go camping I figured the longer wing gets off the ground faster, which is critical for short / soft fields. According to Van's web site, my 135 HP -9 will get off the ground in 500 feet while a 160 hp -7 takes 650 feet. You have to move up to 200 HP in a -7 to match the ground roll of my 135 hp -9. Once I'm off the ground I can always circle to gain altitude but the trick is to get off the ground ASAP.
This little talked about performance item had a lot to do with my selection.
Do I wish I could perform some acro, sure I do but I have a 2G back which limits me to rolls and the -9 rolls very nicely. Oh, did I say that out loud. You aren't supposed to roll a -9.
It comes down to buying and building the plane for the mission you have in mind. The rest doesn't really matter.