Yohimburn is actually a topical fat burner. Prep H just does a temporary tightening of the skin. You hear about competitors, pageant girls, etc using Prep H immediately before a show. Please understand that there are no quickie ways to "tighten" your skin. If you've lost a lot of bodyfat quickly, you have to expect that your skin probably won't immediately tighten up. Generally if you are like under 50, don't smoke, eat a healthy diet, etc. the elasticity in your skin will eventually catch up. As I said, if you are talking about a significant "flap" of loose skin like following a gastric bypass, then you might have to go to the surgery option to simply get rid of the excess skin.
Otherwise, there is some thought that loose skin is simply just the final step of a diet to tighten up all the way. Based on the idea that loose skin is made up partially of water, partially of remaining fat deposits so you just gotta keep hitting the diet.
Personally if you are just trying to lose bodyfat in general and are not looking to accomplish a certain look by a certain date (e.g. a competition), stick w/ the Yohimburn.