Propylene Glycol is thinner and more runny, produces less vapor, but produces a more pure flavor, and has a stronger throat hit.
Vegetable Glycerin is thicker and less runny, produces more vapor, but tends to mute the flavor and has a slight sweetness of it's own, and has little to no throat hit.
Finding the right balance for you is the tricky part. You may want bigger clouds but prefer the more pronounced flavors. You may want more throat hit to feel more like you're smoking, or you may want something smooth to take big lung hits with. 50/50 is a good starting point as you can quickly decide if you want more flavor and throat hit or more vapor.
Also, be aware that few retail tanks and clearos can take high Vegetable Glycerin juices. They just don't have the right wicks for that thick of a juice. Unless you plan on building your own coils and wicks you don't want a really thick juice.