I think both are true. The cardio workout you get from each of them is different and they compliment each other. I do think my endurance on the bike has increased since I started running (and swimming) and running is a great way to stay in shape in the winter when my bike is on the trainer
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"We don't stop playing because we grow old; we grow old because we stop playing." George Bernard Shaw
Luna Eclipse/Selle Italia Lady
Surly Pacer/Terry Butterfly
Quintana Roo Cd01/Koobi Stratus
1981 Schwinn Le Tour Tourist
Jamis Coda Femme