Just noticed there was a question for me, sprry for the delay.
For me the most important part is the visuospatial working memory, I tend to rely very much on it. For instance when I have to figure out how to solve a problem I try to depict in my mind the most important pieces of the puzzle and move/morph them around, sometimes I just stare at my mind images until the solution forms by itself.
As for the above, I would suggest going for visual stimuli being positions in space and shapes. If you want to cultivate you ability to switch intermittingly you can add sounds into the mix.
This is completely free, opensource and you can play whatever configuration you want. It's basically a mono/dual/tri/quad/penta...N-back with configurable stimuli and plenty of settings.
I really don't understand why people still lose time with inferior N-backs being my Hyper N-back the most versatile of them all.