You can choose the name you like, of course, and Brynn is pretty, but it leaves the child few options to adapt it, and no wiggle room for manouvre. I think it is worth having a step back and thinking that the name is the important element of identity belonging, not to you, but to the person who's been named. She will have the job of dealing with it. I knew a Sophia who pronounced it Soph 'I' a, like the capital I in the middle, and she got well fed up correcting people. Having people thinking you were going to be a male, will be a nuisance. There are arguments for playing safe.
It's a luxurious liberty we take as parents to choose our child's name to reflect our own taste; years ago currying favour with older relatives would have been the motivation, or faith reasons. If you are a famous celebrity, you can afford to play creative with baby's name, because you child is going to be so advantaged, in any case, and probably will move in privileged, golden people circles, anyway, with other super-creatively named kids.
Yes, as parents, an unusual name choice for your child may reflect edginess, cool, taste. But parents are not having to cope with the misunderstandings and corrections as the child will, as an adult.
James has soft sound which Brynn just doesn't have.
I think giving an easy name to a child is a blessing.
So, I don't think, at bottom, a friend's concern is about them being rude, I think they are conscious of the child's perspective on being identified with a very unusual name, and trying to be helpful. If they know you well, perhaps they are being reasonable and helpful.
Of course, it is up to you, but I don't think a name is like choosing wallpaper, some place where you indulge your fancy. I think pragmatism has to come into it too.