It's impossible to say which is better because it is a matter of personal opinion. The idea of Dragon Ball Z Kai is to take the best parts of Dragon Ball Z (there was A LOT of filler) and put them together in a better art style (the art "quality" was not as good in the 80's; Kay was made in the 2000's).
The idea was that Dragon Ball Z continually got ahead of its source material, the manga, because it had to make a 20 minute episode every week going along with only 15 pages of manga a week; they would compensate for this by adding in filler episodes to allow the manga to catch back up. So, when they finished Dragon Ball GT, they decided to go ahead and remaster Dragon Ball Z. This time they had all of the source material, and animation technology was better, allowing for a "higher quality" animation. This allowed them to cut out all the filler and just have content that pertains to the story, all in great animation and music.
The reason I put higher quality in quotes is that quality is subjective. I personally like the old style better because I think that the characters are more expressive and the animation has more character; however, the new animation looks really slick and the music is awesome in Kai. It is worth trying out both and seeing for yourself which you prefer, and you can always skip the filler in Z.