I am guessing your question involves over-all health, rather than simply losing weight. To build your wind up (improve your cardiovascular performance), intense exertion over short periods of time, such as sprints, will give the best results. However, for over-all health (weight loss, cardio, muscle tone, mobility, etc...), a controlled effort over longer periods of time give the best results (cross-country running, x-country skiing, long bike rides, etc...), and also offer the longest-lasting results.
My suggestion is that you take some nice long, fun bike rides throughout the week, and on alternate days, do interval sprints, hill climbs, etc.... This will give you the best of both worlds. One of the best ways to get saddle time in is to use your bike as transportation, to run errands, visit people and places, or even ride back and forth to work. This usually will involve both types of riding, and save on your car fuel expenses as well...