Its a great question Kate, and as Caz said, the best thing to do is keep asking questions so you can arrive at the right decision for you.
The way it was explained to me at hospital was that patients *on average* lose 70 - 80% of their excess weight with a bypass, and around 50% with a band. Now of course, the harder you work with it, the more likely you are to lose more than the average , but for me, I felt that there was a chance I could have a band and still be classed as obese when I had finished losing weight. I didnt want to take that chance.