you're right - weight loss is usually greater with the DS, but on the flipside, there's a greater risk of complications with it, too. So....??? Either way, though, major complications with either surgery are pretty rare.
if you have GERD, however, bypass would be the way to go. If you don't, it really comes down to personal preference.
i'd just do a lot of reading on both surgeries. One thing to consider is that pretty much all doctors are familiar with the RNY because it's so common. I "know" people (at least from online forums) who've had the DS who've had to educate their PCP's because not all of them are familiar with it - it's not a very common surgery.
other than that, most people seem to be happy with their choices. I don't think you can go wrong either way.
Edited to add that you're correct - DS is usually done on people with severe morbid obesity. Although I know someone who was around 250 lbs (a woman) who had it done - so they do do it on lighter people occasionally, too.