It depends I guess:
- Tablet: Since you're not looking to use it for anything else other than osu!, how often do you plan on playing the game? Your sig appears to be against this option.
Also do keep in mind that you may not like the experience at all and will possibly have to revert back to using the mouse like some here did. - Headset: Cheaper and will probably be more frequently used - this looks like a no-brainer.