Bron, I've been there too! I remember doing a piece with 2 of my rowing buddiesin college, a featherwiehgt and mens open, at 5 am. It was a 10k, 1 minute at 2k pace or faster and 30 secs off at paddle. I remember finishing and passing out along with my featherweight friend.
But with cycling, you don't pass out. Also, maybe I haven't pushed it all the way on the bike, but you don't quite feel the same pain either. I remember doing 6x 2k at race pace, or better yet, seat racing, where the pain and lactic acid was phenomanal and then fades out b/c your busting your butt to move. On the bike, it's not the same (maybe a good thing!).
I love both sports but right now, biking is much more convenient since I don't have the time to go out and row at the club. I love to bike and having a rowing background really helped me get involved. It's a great cross training tool as well. When I coached, I would bike about 15 miles to ergs, do a 30-45 min piece, coach the practice, and bike back. It definately strengthened my legs.