sammerson:
To learn what each iron carries without any spin. Carry is the "air time" of the shot and is more important than anything else. If you don't know where it's going to land, you won't know how to control what happens after it lands and full back spin seriously affects this carry, not always consistently.
It wouldn't take more than the life of one ball for you to figure this out for yourself. Find a long fairway (like on BPB) and hit up and down the fairway with each iron noting the distance when you hear the thump of the landing. This is your carry number and knowing that will allow you to plan your shots more realistically.
FWIW, full back spin is a terrible way to learn clubs for a number of reasons. For starters, full back spin puts you in a turbo mode for misses-you miss the ding and you get slaughtered. It also lifts the ball and allows the wind much more time to affect it.
I use the 71 Nike mostly because the carry of those Vapors comes pretty close to the rated distance with this ball. It makes it easy to know what the club is going to do and then you can use spin to augment the landing.
Chrisironsbones has a chart for the R11s on his page that will help you understand the trajectories of the different irons. They use the same distances as the Vapors but the 6-PW of the Vapors is what is really different between the 2 sets. The 3,4 and 5 irons are similar but after that, the Vapors run away with performance on the rest of the set.