Both apps were rated the same, about 4 and a half stars. Instacast is newer, so had fewer ratings, but came recommended by a blogger I read. Both apps are the same price and have roughly the same functionality.
In the end, I bought Podcaster because it had fewer typos in the App Store description. Details matter. In my experience, attention to detail is a way of life. It permeates an individual, it permeates an organization. I expect the attention to detail in the app store description will carry over to the app, and so far I have not been dissappointed.