The most important thing with aerobatics is the instructor. The aeroplane is secondary.
Alan Cassidy is one of the most respected aerobatics instructors you will find (unfortunately I've never flown with him, but others here have), and the Pitts is a lot of fun. It also climbs to aerobatic training height very quickly which is useful.
However, having said that, I'm not sure I'd recommnd the Pitts or the Cap initially.
Some manouvers in the Pitts are too easy. You can fly a passable roll just by moving the stick to one side for a couple of seconds. Try that in something with a slower roll rate, and you will quickly realise just how little you know about rolling! On the other hand, if you can successfully roll a Cessna (or a Slingsby, or Robin, or Citabria, or Chipmunk, or... ) I guarantee you'll be able to roll the Pitts.
Similarly the Pitts will fly a passable stall turn with quite a range of entry speeds, whereas a lot of lower powered types need just the right speed and have to be "coaxed" a bit more.
The Pitts has a constant speed prop, which makes life very easy when flying aeros. But in the early days of aerobatics, it's not a bad thing to fly with a fixed pitch prop, and develop a healthy concern for not overspeeding the engine. It's a bit like driving a car with a manual gearbox - you can convert to an automatic very easily, but if you learn in an auto it takes longer to convert to the manual.
As for the Cap, it's delightful - but has a couple of drawbacks. First is the current (and hopefully temporary) restriction of the G limits. You can very easily overstress it, so your instructor will have to take control early if manouvers go a bit wrong. It's not a bad thing to develop a slight paranoia about overstressing, and to always glance at the accelerometer as you pull into a manouver, but in the early days it's perhaps nicer to be able to screw things up without worrying about pulling the wings off.
Second drawback of the Cap is that it's also a bit easy. The roll rate isn't Pitts fast, but it's way faster than a lot of other machines.
The ideal solution, in my opinion, is to find a goodinstructor with a draggy aeroplane that doesn't roll too quickly, but which has a decent rate of climb so you don't waste too much time climbing.
Good luck!