You can be successful with either surgery. Both surgeries require a new relationship with food and good choices for life. To be successful with either surgery you will still need to address any stress/emotional/bad habits that might have contributed to your overeating in the past. Neither one is a magic bullet, but both provide the means for your body to release its fat much easier than without weight loss surgery.
My point is, don't get too caught up in deciding which surgery to get. Some people choose VSG because they feel it is a bit safer, simpler, and keeps their digestive tract in-tact. Others choose RNY because it deals with GERD/Acid Reflux better, has a slightly higher "average" weight loss, and as you mentioned, some people see the potential of dumping syndrome as an additional tool.
Both surgeries have risks, but in the grand scheme of things, they are minimal, but unfortunately some people go through a pretty tough time after surgery. However, at the time they chose their surgery, they were choosing what felt right for them, and that's about where it has to be. Although I'm sure there are some exceptions here or there, people who fail, after either surgery, fail for similar reasons.
Neither surgery deals with our coping mechanisms or makes us perfect beings who always make good decisions We still have to take that on, especially after the effects of either surgery start to reduce with time.
I chose VSG because I felt better about keeping my normal food path, BUT if the surgeon would have given me advice to get the RNY that made sense, I would have done it in a heartbeat.