Your concerns for the particular implant are valid but the difference between these two implant companies is so small that neither one has been deemed better than the other. I think that if an ABPS board certified plastic surgeon has recommended your implants then he's probably right. Remember that the measurements that are made on your tissues (stretchable) are made with a ruler (rigid). Just by the photo that you've posted, and knowing the amount of size difference that you want, it's likely that the implants will be a little wider than what you measure along the base of your breast tissue (I'd have selected the high profile variety). Once they're placed under your muscle, they'll find a way to adjust in there. I suggest you don't go too big, but that's a question for you and your surgeon to decide.
The implant companies have differentiated their numbers so they're not exactly alike but their products are incredibly similar. The difference in measurements is often a milimeter or so and that really doesn't make a difference once it's in the breast pocket. We try to measure everything very precisely, but in all fairness, if two people measure your base width, you're not going to get the same number down to the millimeter. With that said, I use Natrelle just about every day and have yet to find a body type that didn't match the measurements where I wished I'd used Mentor. If this matter is really bothering you, then you should discuss it with your surgeon, I'm sure they'd be happy to spend some more time with you on it.