Having owned both, I think the decision should really depend on how well you land now.
I moved up to a full-sized PZ T-28 too early, before I was very confident at landing and before I could do so consistently. It resulted in my crashing beyond recovery two T-28s. I'm on my third and can now land on a dime and do all sorts of approaches in all sorts of wind and still land. I wouldn't base the purchase on what you can do in the air: we're all good in the air much faster than how good we are low and on landings.
I recently flew my Super Cub again, and I was surprised at how responsive it was and how fun it still was to fly in the air. Landing it, with the ability I have now, was MUCH easier with the Cub: I could land it at almost any speed and stupid approach I could muster. The aileron training on the more stable Cub will help a lot to your getting used to the T-28.
Even after you get your T-28, I would recommend you warm up with the Cub as your first flights for the day, to calm yourself down and to ingrain the basics of flying and especially landing, before you fly the T-28. Even the stock T-28 has a lot more power and can get away from you in the air much quicker. I have some suggestions for newbie pilots on how to land in my RC Groups blog
, but your choice if you wanna read it or not.
I like getting new planes like the next guy, but if I had to do it all over again and was going to get all the planes I was gonna get now anyway with the greedy eagerness that I have for shiny new planes, I would make the aileron mods to the Cub first then get the T-28. I like the Navy T-28 version, as it is much more visible in the sky. It does kinda look like a sissy on the ground though.