Horn or Ribbon tweeter with fullrange

[sharptouch]

whats the differences besides sensitivity.

for instance, Fostex T drivers like the 500mkII vs. RAAL

[badman]

Either a horn or a ribbon should have a limited dispersion pattern, which is useful for supertweeters where the fullrange is already limited in beamwidth. Which is up to you. The horns will (typically) be more directional and more efficient. Beyond that, generalities fall apart. I'd say choose which seems to meet your spec best, there are many excellent drivers to choose from (though the RAAL does seem to be a superb device by all accounts, I've heard it once and the overall speaker design was trash, so no help there!)