I'm building the 9 and think it is the great plane and, if you're willing to forgo aerobatic potential, the best of Van's line... That said, however:
1. The -9/9A is still very much a "hotrod" compared to any production plane that one would be likely to train in. It can build up speed REALLY fast if the nose is allowed to drop at all. Van's demonstrated this to me very dramatically on a demo flight! Compared to a 150/172, a student could easily get themselves in trouble. Well trained they'd be less likely to, but student pilots are likely to make mistakes. I kind of laugh when it's referred to as a "trainer." The words "trainer" (and all that connotates) and "RV" just don't go together (fortunately!
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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2. Learning to fly in an RV could help you develop some bad habit regarding rudder usage. RV's (and perhaps especially the -9) can almost be flown with feet on the floor. Learning the NEED to use rudder rather than aileron to pick up a wing during a stall in a 150/172 was one of the best things the 150/152 taught me. You don't need to do that with the -9/A because the stall is so benign and the ailerons so effective. Coordinated aileron/rudder usage likewise. If you have a good instructor who can find a way to instill this rudder usage into your training in an RV, this issue would not be as important.
3. Early landings? I think I handled the local FBOs trainers relatively gently, but I bounced my headset off at least once. Do you want to do that on a plane you built (or spent a LOT of money to buy)? Moreover, I think there is ENOUGH question about the nosewheels of RVs (as someone above mentioned) to make you think twice about this. I'm not saying there is a "problem," just that they are probably just not as forgiving of the very poor piloting that all student pilots do at some point in their training.
Again, good instruction can probably overcome the first two, though probably not the third if we're dealing with student pilots. All in all, if trying to acheive cost savings, buy a 150/152/172 for training purposes and then sell it when done and buy an RV. You'd probably sell it for what you paid for it, making the flight hours free.
Hope this helps your decision making. FWIW. Good luck.