S.B.
I had my breasts done about 10 years ago, and I don't regret a second of it :) I decided to get them done for much of the same reason that you are considering it - I felt "unbalanced" between my top and my bottom. I was a 32 A and I am now a 32 C/D (depending on the bra maker). The one thing that really helped me decide to go no larger than a large C was finding a 32D is difficult and a 32DD is nearly impossible unless you are special ordering. I was also concerned that if I gained weight as I got older, my breasts would gain weight as well, pushing me to a cup size I wouldn't want. I would highly recommend the submuscular implants versus the ones that just lay on top of the muscle. The healing time is alot longer with the submuscular, but well worth it - they look and feel alot more natural. I have my incision in the crease where the bottom of my breast meets my rib cage and you can barely notice it at all. However, if I had to go back and do it again, I would pick through the armpit or bellybutton if the option was available (I decided against going through the nipple back then and still wouldn't today). No one would ever notice my scars, but I do :) I would say that the only downfall that I have experienced is some loss of sensation in the nipple and around where my scar is. The chance is there, I was well aware of it, some people have more or less feeling loss than others, mine is on the higher end of the scale.
My only other advice is if you are thinking of going to a small C/large B, I would go bigger. They say that over 80% of women who have implants wish within 2 years of their surgery they would have went a cup size bigger. I think that we tend to not want to go "to big" because society tells us that is "slutty" to wish for big breasts. I am certainly not saying that you should Pamela Anderson yourself - just don't set yourself up for disappointment.
Good luck!