Both have a demo, so you can try them out. What really bugs me is that the Soundtower edition is limited in features, so there's no way of knowing if it works like your would expect it to. Codeknobs on the other hand is fully functional, for at limited time.
Sequencer in Soundtower looks fastest. I like how you can just 'draw' the notes in there. In Codeknobs it's a slider.
Overall codeknobs has some really cool features, like parameter reset to program default with cmd click, and tweaking just seems much smoother. I just wish it had both layers on the same page, stack a/b indicator, clickable wavetable selector and clock sync time signature.