Yep, I have experience in both, I'm the programmer of Yankadi

As you may know, the program has not been updated and is no longer under development. You can still download the shareware and use that.
It had some unique features such as the introduction of 'swing' during playback, normal music notation, a layout that was geared towards printed music instead of the piano-roll that Percussion Studio has. But I also ran into technical problems with playback since I had to rely on Microsoft's DirectX engines, and micro-positioning of notes was not possible.
Percussion Studio has drawbacks too, although I use it from time to time. PS would benefit greatly from improved freedom to position notes, from improved keyboard control, and from some sort of 'overview' window (or a zoom function) so you can see where you are without having the large notation scroll away all the time. I've approached Henry Kellner for this in the past without any results.
My own efforts at the moment, in notation, are purely towards font development for djembe notation. I have one font that is inspired by the font in Mamady Keita's book, and I am currently developing a font that is inspired by Percussion Studio, which would make for a more condensed notation that you can still read from a fair distance, allowing for cheat sheets on the floor for example.
See my site
www.yankadi.silvercircle.org for all this stuff. Well, the new font isn't there yet of course but the other one IS there, and is also being used by various djembe book authors.