Stationary bike vs. on the road

I ride a stationary bike, either the more bike-like "upright" one or the more comfy "recumbent" one, two or three times a week through the winter - less often in the summer. There are certainly differences in which muscles get used but I believe I get a good cardio workout and build general muscles that I can later tune for use on the bike - when the weather is more conducive. I keep the RPMs high (100+) and tend to do "Hill" programs that have intervals of hard work. I'll typically do 45 minutes sessions at levels 14, 15 or 16 (out of the 20) on the bikes at my gym. YMMV.

I'm way too far from being an expert to try to make any worthwhile objective physiological comparison and, frankly, I'm not real interested in trying to hard to defend what I do. I do it 'cuz I enjoy it.<br><br>
But I know this. I most often spin at lunchtime on the days I've run in the morning - like today. I breathe hard, my legs burn, my heart rate gets into the 130's, and I leave a puddle on either side of the machine. I could alternatively sit at my desk over my lunch hour surfing running forums... I think I get a better workout on the stationary bike.