I go to a gym that is computerized and keeps track of your workouts. I have been going for about 2 years and 6 months. I just went over 20 million pounds lifting. I have about 5000 miles on the spin bikes and 450 miles on the treadmills at 10% grade.
My conclusion is that riding helps the gym more that the gym helps riding. Spinning at high cadence has helped up my riding cadence. I don't think high resistance on the spin bike is the place to build strength. My legs are much stronger but I set the age group record on my first strength test so working on strength when you are already strong helps the riding only a little. I do 4 set of 30 on most equipment. I started with low weight and increased slowly but it adds up after a couple of years. I started doing 5 sets on leg press because the weight was high but I'm back up to 420 pounds now. Treadmills and spin bikes are better than nothing but are not even close to the cardio work out of riding a bike.