statistically speaking, you'll lose more with bypass than you will with sleeve, but it's only about a 5% difference. Plus an average is just an average - you'll find people who fall on either side of the average. There are people who are extremely successful with both surgeries - and people who fail with both surgeries.
14 lbs is a lot. You may lose that the first three or four months, depending on a whole host of other factors (sex, age, metabolic rate, activity level, starting BMI, etc), but weight loss slows down a lot after that. I started at well over 300 lbs. I lost 16 lbs the first month, maybe 12 or so lbs a month for a couple of months after that, then I think around 10 lbs a month until I was about 7 months out, and then 5-8 lbs a month until I got about a year out. After that, things really slowed down until it finally stopped at around 20 months out.
P.S. With your starting BMI, your weight loss isn't going to be really speedy. You WILL lose it if you're committed to your program, but 14 lbs a month is overly optimistic for someone at your weight. Most of us start out much heavier.