I use both a prism and the WL finder. The prism is nice but not necessary if you can learn to accommodate the laterally reversed image. I actually use the WL finder most of the time as I usually use a tripod or monopod. The winder would be nice to have but, again, is not necessary as you will be most likely working at a slower pace. If you decide on either be sure to check ebay and other used sources for less expenditure.
Others may have different opinions. It really depends on what you want to do with the camera.