By far the former is better since you can get a feel of what it's like to work at a particular place. Looking in the Monday edition of The Japan Times, which has that week's classified advertisements, is also one good place to start. You'll probably be surprised at just how small many of the bars are. Some seat only 10 customers or so. There are several positive aspects to look at.
Work is relatively easy to find.
It's also a great way to pick up some of the language quickly. You should know at least a little Japanese before you work, but unlike teaching English where many managers frown or get in a huff if you speak some Japanese, the more Japanese you learn, the more money you can make. The Bad Needless to say, there are lots of negatives to think about also. At best the work you can get is part-time only, which means you can't get a work visa through it. Unless you get a visa someplace else, like teaching English, going to a school, etc., you'll lose everything in a heartbeat if the police come, your mama-san decides to screw you over, or even worse, if the mafia are in any way involved.