Yoga vs. weights ..what is best for swimming?

I just ordered and received my "Yoga for Swimmers" DVD. I have watched it a few times and attempted many of the poses....it is harder than I thought and I'm way less flexible than I thought.Can anyone out there attest to the benefits of yoga when it comes to swimming? I'm trying to decide whether to do a consistent weight lifting routine or if I should practice yoga - I don't have time for both right now.