My 2 cents
Don't underestimate Arduino , it has good points, don't care about the critics, you can learn a lot with it. I learned (and quickly than PIC/AVR alone) ! And remember you can always build your own version, it's open source.
Try Both AVR and PIC solutions, pick whatever you find most suitable for you or use both. MPLAB X is good as AVR Studio is. Both are not "the best" but they are "the best free available" and for start to play around that's quite enough.
Start small and scale after. As you are quite proficiency you won't take too much time to learn the basics. As you are also from old school, a book is always welcomed. I liked a lot Designing Embedded Systems With Pic Microcontroller. It uses a 16F84 which maybe not the best choice by now but which will guide very well.
Regarding ARM, my opinion is forget it by now. Start simple, build your projects, and then go ARM. Don't go straight to ARM, nobody deserves this !