An SLR can have a waist level finder as well, so that's not really the axis of comparison. Also, some TLRs can take a prism for eye-level viewing.
In general, a waist level finder will be brighter than a prism finder, because the prism eats light, but a prism does a better job at blocking out extraneous light than an open waist level finder, so it might be easier under certain conditions to focus with a prism. In low light, you'll probably find yourself using the magnifier with the WLF, holding it at chest level, so that your head will block most of the extraneous light, and under such conditions, it can work pretty well. It will depend on the quality and type of the focusing screen (often upgradable, even on old cameras), and the condition of the mirror (old ones can be dirty, tarnished, or worn).
A nice thing to have, if it is an option on the camera you choose, is a chimney finder, which is just a simple magnifier that blocks out extraneous light. These have been made for SLRs and TLRs.
Waist level finders are pretty stealthy for street shooting and candids. On the other hand, you can always pick out a wedding shot with a waist level finder camera, because you can see up everyone's nostrils.