Hi, Randy - Welcome!
I personally have only been around male GSD's (we grew up w/intact males out in the country as kids, and now I have my own 8 month old male GSD pup). All the dogs we've had in my family (we are GSD lovers!) were wonderful. All of our males were intact outdoor dogs - great w/us (as kids). My male GSD pup is great w/my kids (3 kids 6 years old and under). I know you don't have kids, but if you plan to, it's nice to know that GSD's can be great kid-dogs.
Anyway, when I researched getting my own pup, here is what I heard pretty much from everywhere that stated that there was even a difference between males and females (some people who've owned both said no difference between the sexes). From what I've read - males are more "territorial" - protect their home, their territory (inside their car, yard, etc). Females are supposedly more "family protective" - they focus their protective instincts on their "people" vs. their "territory." I have no idea if this is true or not. Our males were always protective of us (very
protective of us kids) AND our home. Never aggressive, but would warn strangers "don't get out of your car until my people say it's ok" type of thing. They always would stand between us kids and visitors - even if they were smiling and happy, they would stand between us and people who weren't part of our family pack. We always had about a dozen cats (our cats kept the barns cleared of mice, or at least, that was the idea - lol.) Our dogs and cats co-existed (outdoors and with no intervention from us) just fine.
Don't know if this helps or not, but this is what I've heard...one thing I'd suggest w/a GSD - is they were developed to be herding dogs, and they are one breed who do better with some type of "job." It doesn't have to be a formal job, or competitive sports, but even if you just incorporate obedience training into his everyday life (my dog has to sit to get his dinner, or before I let him go outdoors with me, for example) - obeying your commands could be his/her "job". My dog is training to be a therapy dog, and so I do more obedience/socialization that the average pet owner, probably, but I would still incorporate obedience into everything we do, even if he wasn't going to be a therapy dog.
If you use the NILIF philosophy (Nothing in Life is Free) with your dog, you'll be fine with either male or female, IMO. Sorry this is so long, but this question was one that I had when I looked at getting my pup...hope this helps! Good luck!
Oh, and as far as the cat - if you get a fairly young pup, you probably would be able to get the dog used to accepting the cat. Like I said, GSD's are a herding breed - they are not bred to "attack" or fight with other animals - your dog may even end up protective of your cat! Maybe start out by exposing your potential dog to a cat who's in a carrier (nice and safe), with the dog on a leash, and see how the dog reacts. I'd be willing to bet that the cat will be the one who doesn't like the dog, not vice versa.