I'm thinking of doing a Tri myself... however I've only been biking and running so far. I think swimming is better than running is better than biking in terms of fitness training. It makes sense, because you're using alot more muscles in swimming vs running and running vs biking. I think cardiovascularly you definitely get more out of running than biking, because as has been noted before, biking heart rates are lower than running heart rates. As for me, I train at 180 bpm running, and ~160 bpm biking. I definitely feel more winded after a long run than after a long bike. My legs tend to be more tired and sore (muscles) after a ride, but I am sweating more profusely and breathing much harder after a run. So I would tend to agree with your wife .