Just in terms of losing weight - all that matters is calorie expenditure. So there's no difference - you need to work out approximate KCals burned per short session, add them up and compare that total to the total KCals burned in your single longer session. If however you're training for something specific: Long rides are good for endurance and a general increase in fitness (in terms of resting HR, Max HR etc and also in terms of body adaptation to cycling. This type of fitness is slowly built up though - think years, not months). It is not good for increasing speed or strength particularly - although of course the more you cycle the more these will naturally increase anyway within reason. However, you are more likely to wear yourself down and get ill riding long distances (especially if not built up very gradually) in all weathers. Speed/strength work does exactly that but wont increase your base fitness beyond a certain point. All types will burn KCals which is what you want to lose weight. I suggest that you need to do both types for a healthy balance. Most people use steadier base miles to keep fitness ticking over through winter than training for something specific in spring using strength/speed work or more specific training for their chosen event(s).
You say you have reached a plateau in terms of weight loss. This is common and means (assuming you haven't started eating more KCals than previously) that your body has adapted to the training you are doing so needs fewer calories to do it (ie. you have become fitter - which is a good thing). Therefore you are going to have to up your training levels in order to loose more weight. This can be done either way - more short sessions than you currently do, or, longer rides. You may also want to look at your diet and see if there is any excess KCal input that you could cut out.
