I am an owner of an IRO Mark V and I'm very happy with it. (although I still think it was stupid to pay so much for the "custom" options.)
I've been riding it for about half a year now, and nothing bad has happened to the frame (even after 3 crashes, one more serious at high-speed). Everything is still intact. I basically used it for commuting and occasionally skids. However, I can't speak on its performance if your're FG freestyling with it.

But if you dig in the forums and read some more, you'll find that there is no weight advantage between an IRO Mark V vs the Pro The ride quality may be improved however, like PedallingATX mentioned.
I would pay the extra $50 to get the Pro just because Reynolds has a very reliable reputation.

You should also consider the Angus, geometry should be the same, but imo, looks better because it's 1" threaded so you can put on a quill stem, and has no cable/ water bottle bosses (pro version available too).