If you have a heartrate monitor the Garmin numbers are pretty good as they license a pretty nice algorithm for the calculation. When I got a powermeter, my reported calories for my workouts stayed nearly the same...that is, the HR based estimation nearly matched the actual work calculation from power meter data.
If you don't have a HR strap, then I'd go with strava's numbers. They seem better at calculating work done, and thus, calorie burn.