Both. One burns more calories riding far, fast. However, as above, intake is more important. Smaller portions works for most folks. Same food, less of it. Simple. What used to feed us 2 nights, now feeds us 3.
Looking at the data, losing weight slowly as a result of lifestyle change works better and is more likely to keep it off. Ride more frequently. On long rides, eat on the bike - you'll eat less when you get home and less in toto.
How to ride far fast is a training issue. But if your time and days to ride are limited, you'll have to settle for fast. If your time is not that restricted, 5 days of 1.5-2 hour moderate rides, plus one long, hard weekend ride is a good combination. "Moderate" rides are rides which you can do every day without needing a day off. They are not easy rides. One pushes hard enough to tire the legs every day, but not so hard that one can't do it again tomorrow. One gradually hardens up so that one can push a little harder, further. Takes a few months. I used to ride with a woman who did a 55 mile round-trip somewhat hilly commute which she rode every weekday it wasn't icy or snowing. She lost weight like crazy, got fast. I expect it took her a while to work up to that, but I don't know the details.
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Results matter