Well seeing as you have played baseball, you know that, despite what Evadman does, swinging the heaviest bat, more times than not, makes you look like a moron. Since it is slow pitch soft ball you may be able to get away with a heavier bat since you have pretty much all year to decide whether or not you are going to swing at the pitch as well as executing the swing.
You also need to watch out about length. Make sure the Length:Weight ratio adheres to specs set by the league. I would think that with slow pitch softball it would be a -5, but it may be a -3.
As for metal or wood, it is really what you feel comfortable with. Metal will go farther and are generally favored if given the choice, Wood really isn't necessary and will hamper the distance you can hit the ball.
I haven't found a place that will let you try out Baseball/Softball bats before you actually buy them (Golf really has it beat on that), but you never know.
-Kevin
(Oh and remember to buy a Softball bat. They barrel remains almost the same width throughout instead of increasing in width like a baseball bat)