"if you want to improve your English" is better, as it implies that the purpose of the podcast is to help you improve your English.
To say "if you're going to" implies that the podcast might be useful in some way, but not in actually doing that thing, ie improving your English. Instead it implies that the purpose is to help with something that goes along with, or follows on from, learning English.
Think of "Download this podcast if you are going to France". It's not saying it will help you to go to France, it's implying that it will be useful once you get there - perhaps it will tell you some places to visit, for example. On the other hand, if you saw a podcast saying "Download this podcast if you want to go to France", then that implies it's going to help you to go to France.