I would prefer Twitter as Twitter is having Good quantity and quality of users and also Twitter got compatibility with so many of other promotional things like blogger, facebook, myspace, etc. Twitter is also having other application adapting with allow us to promote induvidual twits by sharing. Also, if we keep all these functionality aside, Twitter is more of youthful. I love it...
I can't stand twitter and I have not heard of jaiku before, so I use none. Maybe oneday I might see the point in micro blogging.
I like Twitter. There are so many new social network trying to compete it's really hard when everyone is already set using it to change. I don't think they will last long.