I would say do both if you can. I have been running when I don't have time for the 1.5hr+ bike ride. After biking for nearly two months and not running for that amount of time, I recently did my usual run around the block 2.5mi. I ran it just as fast as the last time I ran (before I bought the bike in July). So I did not suffer any set back. The next time I added some miles and ran 3.8 with no issues at all. Going to go for 5mi next time I run and see how that goes. I think cycling for extended rides has given me greater stamina for running as well. YMMV.
Wow, I just noticed this was from 2002 after posting......