I don't agree that a 9 inch is cheaper to build. You should probably change out the standard center section on your ordinary 9" so you would need aftermarket center section and pinion support, after that the bearings, axles, housing ends, spool, gears are not that much different in price.
That said I think 33 spline is the largest you can put in a factory 12 bolt. I will say that in my experience it is easier when modifying 9" rears and as Brian said easier to work on. Ultimately the 9" can be made stronger. I wouldn't spend the money to change out the 12bolt in my roadster but if I were starting from scratch I would go with a 9" because of the options it presents and for resale value.
Curtis