I had the bypass. And I have to say I think diets don't work, because we're biologically wired not to stick to them. Chemically our hormones have far more control over us than we realise - I don't think anyone can understand the compulsion to eat that we are sometimes faced with if they haven't been there themselves.
Personally, I love my bypass. I do have a limit with sugar and can dump if I don't check labels, but usually I can eat whatever I want so long as I'm clever about it. I work pretty hard at it and am super happy with my losses today
I also was told that the bypass was the gold standard, but bear in mind part of that is because its the longest available and the surgeons will have perfected their technique. I do feel that the difference between the sleeve and the bypass is not as big as we are led to believe - the difference in losses tends to be an individual thing. Part of it is how committed we are to the lifestyle change, and part is how much our bodies are prepared to give up in all honesty. Our bodies are designed to store energy :/
It might help to do a weeks honest food diary and show it to your surgeon and discuss it with him. He will have seen hundreds of patients and will be able to best advise you.
But for me, I couldn't bear to be fat anymore. I was so ill by the time I agreed to go for wls (my gp had suggested it for a while) that I went for the most final, most drastic option available. In fact, I asked for the DS but was told they didn't do it because of nutritional deficiencies and the bypass was my best option.
As Fiona has said, ultimately its your choice. Whatever you go for, be confident with it and work with it and you will lose the weight.