Dave, either approach is viable.
If the server-side form processing is similar for the various situations, I think it'd be best to set a hidden parameter on the form that the servlet (or whatever's doing the form processing) can use to adjust the processing accordingly.
If the processing will be different for each case, you can modify the action attribute of the form to specify completely different servlets.
Which approach you take depends upon what makes the most sense for your app.
In either case, this is purely an HTML/Javascript operation, so we might want to move this dicussion
here.
hth,
bear