As Trillian says definatly go with your friends kittens, you will know that it has been well cared for and you get to watch it grow which is fascinating. Also this give's you a chance to bond before you take it home.
Failing that i would go to a rescue, so many kitty's need genuine loving homes. and you know that you will have a healthy happy kitten or two (alot of rescues only home in pairs, But not all)
There are plenty of kittens "for sale" but i have noticed that some people are charging silly amounts for moggy kittens just to make a few quid!!! Plus you don't know if they have been cared for properly.
If you decide to buy one elswhere make sure that you ask lots of questions about temperament, treatments and care, make sure you see the mum and the house (i wouldn't arrange a meeting anywhere to pick it up) but having said that The seller does not always tell the truth and you could end up with a very poorly kitten.
I have kept both males and females and really i think it is a personal preference i currently have all boy's. As long as you have them neutered/spayed they will grow to be happy cats. Problems can occur when you dont have them done, Especially with male cats as they are always in competition with other un-neutered males. They will spray and could become aggressive. Also not to mention a male will not think anything of travelling miles looking for the females so you may find one day they have upped and gone, never to be seen again.
I also find that a females temperament can change if allowed to have kittens as she becomes fiercly overprotective and can become aloof.....but this is not always the case, just stating what Could happen.
So it depends on what you are looking for in a cat. It sound like you are not too sure at the moment so i would wait until your friend's cat has kittens and see how it goes as you will have at least 8-12 weeks to figure out what is right for you.