I've always prefered male rats.
    It would seem that I'm in the minority in this opinion; from a couple of different internet polls, most people prefer female rats, and most people keep female rats.
    Some of the reasons are obvious; many people find the males just a bit unsavory.  The you-know-whats hanging between the legs scare off many people.  The scent-marking habit makes even more look away.  And the few that are left, may be put off by the males' overall coarser looks; larger size, harsher, greasier fur.  They just aren't the gentle, sweet, fawn-like (okay, maybe that's pushing it) bodies the females are, at least not in looks.
    But for me, although I'm no huge fan of any of those things, their personalities will always make up for it.  I've always prefered cuddlier, more laid-back animals; male rats are very much that!  Say I'm sitting down holding the rats; my average female is going to be sniffing here and there, exploring every corner; if she's really hyper she'll practically be bouncing off walls in her enthusiasm.  My average male is going to be sitting in my lap, cuddled in my hands being scratched.  That's just typical.
    Now, don't get me wrong.  I love the females; I love their zest and enthusiasm.  Some of the calmer, cuddlier females make ideal pets for me because they're about "mid-range" between males and females without some of the males' bad traits... on the other hand, I've known a number of females with at least some of those bad traits (notably the larger size and scent-marking; I've known a few females who could scent-mark with the best of the boys!).
    But, in the end; if I had to choose only one sex, it would be the guys.
    Going beyond the personalities, into the more practical differences;
    Yes, the males scent-mark.  This involves leaving a small amount of urine over everything, to a greater or lesser degree.  Some males do it more than others; some males I've known barely scent-marked at all, and as I've said, I've known females who did it all the time.  But your average male is going to do it considerably more than your average female.  Most people are automatically disgusted by this, but it really isn't as bad as it sounds; it's only a small amount, and it's easily wiped up.  You become used to it after a while, although I would suppose the females would be better for true neat-freaks!
    Yes, the males have the you-know-whats between their legs.  Males can be neutered, though of course that seems awfully unnecessary if you're just doing it for the visual appeal!  I would think that would also lower the amount of scent-marking, although again, it seems like an awfully unnecessary surgery for just those reasons.
    And yes, the males are larger, and their fur often feels "dirtier" because it's coarser and oilier.
    Even if you don't mind these things, other people might; I'll be the first one to show off my females to non-rat-people, in hopes of "converting" them to being "rat people", but hide my males at the same time... at least until they're used to the whole idea!  Males just tend to more closely fit the image the average person has in his head when he hears the word "rat"; that of a dirty sewer animal. Particularly large agouti (wild-colored) male rats!
    Do the males fight more?  People tend to say so.  In my own experiance, they fight under different circumstances; the males tend to fight after being introduced; the fighting will continue until a clear "pecking order" has been established, and then it will stop until another rat is introduced.  The females tend not to fight at introduction, but often have less-serious quarrels fairly often after becoming friends.
    In any case, it's very much subject to personal opinion, and people tend to take sides quickly and almost religiously... personally, I do not consider myself one of those people despite my enthusiasm for the males - I just think they need some more people holding up their side, as they've been getting an awfully bad rap!  I love both sexes; I'm a bit biased towards the males, but that doesn't mean I love my individual males more than my females.  It's just my opinion that despite a few drawbacks, there's certainly nothing wrong with them and in fact have a few advantages; and if people would consider them more seriously they might just find that they prefer them, too!