I think your nutritionist is misinterpreting the data. It's not that the bypass is NECESSARILY more effective long term, only that we have more EVIDENCE of it being effective long term. The sleeve as an individual operation is a relatively new procedure, so we just don't have as much long term data either way vs the RNY. But the info we do have is very promising.
I've done a lot of reading and from my (layman's) understanding, the bypass will help you lose weight faster (6mo to a year) vs the sleeve where you typically lose slowly over a period of 18 months. The typical difference between loss I think is often 10% of your weight ON AVERAGE, which may or may not be a big deal to you. (It wasn't to me, which is why I went with the sleeve.) Regain typically comes down to compliancy post-op more than what procedure you get. The average losses comes from the people who lose 150 lbs and completely overhaul their life and the people who eat around their surgery and lose less than ten or go on to gain past their original weight, so honestly I really didn't take it much into account when I was picking my procedure. I was much more concerned about the life I'd be living post op with both procedures and about my individual comfort level with the concept of the procedures themselves. Sleeve fit my vision for my future, so that's what I chose even though my surgeon was more in favour of the RNY as the "gold standard".
It is true the RNY is more invasive and also produces more restriction both in terms of volume of food AND type of foods you can eat. A lot of the things I can very easily eat post-op with my sleeve could make your average RNY patient very sick. some people see this as a pro to the RNY, others a con.
There are good reasons to pick either procedure, and sticking to the plan long term will bring success with EITHER procedure. After that it comes down to other factors: if you have GERD, if you have type 2 diabetes, how much weight you have to lose, how comfortable you are with malabsorption of nutrients, the speed you want the weight loss to be, whether you'd prefer the proven track record of the RNY, whether you want the ability to revise down the line, etc.
There's a lot to consider here, so definitely stick around and do some reading, but don't let the nutritionist tell you that you HAVE to get the RNY just because "after 5 years the sleeve stops working" and accept that as gospel or use it as the only deciding factor in your choice. Any surgery can result in weight regain, just as any surgery can result in stunning lifelong success. The specific surgery itself isn't nearly so important as the lifestyle changes you implement alongside your chosen procedure.