It comes down to how both procedures work, and is often misunderstood.
The band is a restrictive tool. It restricts the volume of solid food you can eat, by making the stomach into a bottom heavy hourglass shape. Some people feel that if they eat healthily, but just eat too much, then the band will be enough for them. Many unhealthy foods will slide through the band, meaning it requires much more willpower and self discipline than the bypass, and is easily cheated.
The bypass is both restrictive and malabsorptive. So like the band it restricts the volume of food you can eat, your new pouch is approximately the size of an egg. Some foods empty out of the pouch faster than others, such as soups and more liquid foods, but there is a limit on how much of anything you can eat.
It is also malabsorptive, meaning that you only absorb a percentage of what you eat. This percentage varies from person to person and has been quoted as anything from 30-70%. As well as calories etc, this affects your absorption of vitamins and minerals, hence the need for supplements for life.
On top of restriction and malabsorption, bypassers are prone to consequences when eating fat and sugar. Sugar (and refined carbs) cause dumping syndrome, as the food goes into the intestines very quickly and lower down than normal, the intestines fill with water, which makes you go very dizzy, light headed and nauseous. Not nice. Too much fat can affect you in the same way, or can affect your bowel habits a few hours later. Every person has a different tolerance level. Some don't dump at all, although this is rare.
So people who feel that their diet is not the healthiest, that they have a sweet or fat tooth for example, may feel the bypass is more suitable. The bypass also helps volume eaters though, so I'm not sure what the poster said that because she also ate volume she would be better with a band.
HTH
Shel