Your choice will boil down to how much time you want to spend programming it. The Harmony will be quicker and easier to program but the Pronto is more capable yet will require more effort to get the most out of it. If you're happily using a Harmony One then you'll probably feel more at home sticking with the Harmony line.
You can obtain the Pronto Professional 3.0 programming software (Windows only) from the usual "remote place" and give it a try to get a feel for how you'd program it. The Pronto really comes into its own when you start using the Prontoscript programming language (to communicate with networked devices, for example) - if you just want a touchscreen remote that looks good and is easy to program and operate in a "normal" living room setup, the Harmony is probably the better bet. If you've got lots of kit and need some specialist control - RS232, X10, network and the like and can stomach the learning curve, the Pronto is (still) a great bit of kit despite being retired by Philips. If you're a "tinkerer", it's great fun...
For what its worth, I have (and have had) pretty much all the Prontos (including the 9600) over the years and would consider myself a skilled/experienced programmer of them (which includes Prontoscript - I'm a professional software engineer, by trade) but the demand for family-friendly controls and quick/easy programming means that I actually use a Harmony One for day-to-day remote duties at home and my Prontos have mostly been retired into "back-burner" projects (I dream of completing) in my "spare" time.
I'm pretty sure that the Pronto software won't run on OSX. You would have to run Windows on the Mac (Bootcamp dual boot or Parallels-type virtual machine), or use a PC.
The Pronto software isn't difficult to get started with, but it could take a long time to explore all the possibilities. It's reasonably user-friendly.
Prontoscript is a different matter, but you don't need to use that to get the Pronto to the level of a Harmony.
Not sure if things have changed now, but I downloaded and ran the editing software with no problems. The only thing I wasn't able to do was upload the configuration file from my Pronto to my computer. Since I made the config file myself, this wasn't a problem.