Personally, I think riding a bike is better on several fronts, not the least of which is mental exercise and wellness.

I find stationary anything boring as hell and prefer the challenges of riding on the open road. But that's me, and I have time during the day (lunch precisely) that I can realize this as well as many days of dry weather, so only on the random rainy days will I feel the need to hit the stationary bike in the gym at work. Again I am lucky in this regard, and given my often lack of zeal to exercise it helps that I'm not forced to use the gym much because excuses can be all too easy to find ;-)

That you can get 20-25 miles in an hour is nice, maybe if I keep at it I can average that by the time my AARP card arrives lol