Both sexes are excellent choices, just make sure you get a same sex pair as rats are very social and you do not want any unwanted or unexpected litters. I have both bucks and does and have found both to be very personable and friendly and crave human attention. Boys do tend to be a little lazier and chubbier and much more willing to cuddle, whereas girls tend to be a little bit more on the hyper side, wanting to explore and run around more. However there are lazy girls, and hyper boys too! It all depends on the individual rat.
Rats can be leash trained however it's been my experience that most do not care for it. Their necks are pretty small and it's hard even getting a ferret harness and lead to fit one of my big guys. It's not something I recommend. Rats are very content shoulder riding though and love to hang out with me while doing my daily chores etc. They need a lot of out time, preferably at least two hours each day. If you are a very busy person (ie in school) then I would advise waiting until you have more time to devote to them. I am a stay at home mom and lucky to be able to spend time with my furries all day long. Any other questions?
Some people do say that males mark more. Personally none of my males overly mark their territory and the rat I have that pees the most is female. She will pee on everything and anyone to mark them. Males do tend to be just a tad bit stinkier though. However most of the boys I have had have done fairly well in keeping their living quarters clean.
