Have both... not used neither much.
Tripod is definately better for long exposures.
However, security etc in malls/other areas don't like the space taken up.
In comes the monopod. Doesn't take up space, you can use as a grip to teady yourselve. You can wedge for those creative angles, or steady more in macro. But.... I still don't use it.
Probably have their uses and if you don't have 'em you aren't missing much.... I am using my bag more as a bean bag to wedge the camera on for long exposures. Gets lower too.