My Dad's from Ohio, so I grew up watching the Brown with him and naturally became a fan. I was a big Kosar fan.
The Panthers came along when I was 12. I got a t-shirt and sweatshirt with the Panthers' logo on it for my birthday the year they were announced, and have been a Panthers fan ever since.
I guess I can say I'm a lifelong Panthers fan, but it would be the Panthers life and not mine, haha.
So in summary, Browns fan because I didn't know any better, Panthers fan by choice.