The crucial thing is not your riding strategy, it is that you consume fewer calories than you burn. So whether you ride hard or steady, you need to keep track of how many calories you are burning and make sure you don't eat so much - exercise makes one hungry - that you negate the effect. Personally I favour the longer steady rides for weight control purposes, because I seem to get less ravenous that way and can manage my appetite better. Your mileage might vary. With a power meter you could calculate precisely how many calories you burned on a ride. A rough estimate would be about 30 calories per mile.
As for mixing in one or two more intense efforts each week, that is certainly a good training strategy, but that's more about fitness than weight loss. Either sort of ride will help with the weight as long as you manage the calories in<calories out equation.