I know it's kind of off-putting to treat two Catholic media apostolates as though they are competing against one another for airtime, but I just have to ask the question: Which do you prefer? (obviously if there's some other media outlet I'm not mentioning, go ahead and consider it as well).
Here's my analysis:
Relevant radio offers a Talk-radio experience seen through a Catholic lens. They are also essentially mouthpieces for the bishops (and this could be good or bad).
EWTN is much more about Catholicism itself, and is very independent of the bishops.
Of course, they both share a lot of the same people. I tend to make use of both.