It's only possible to be pretty general about this, since it's hard to define when one gets "concerned about" something to what degree. Having said that, I'd proffer the opinion that unfortunately women are more concerned with what they look like, and that it is societies fault for pressuring them too much by setting ridiculous and impossible "standards" of beauty. There's a hilarous cartoon, if you have google you could google it (i'm in china so google doesn't work well here). It's a fat man looking in the mirror, and he sees himself as an adonis. There is also a slim and beautiful woman looking into the mirror, but she sees a bit fat woman in the mirror. I think this typifies the attitudes taken by many.
On the whole, there is much less pressure on men to look awesome, even after being fashionable and stylish became a straight thing (thanks to people like David Beckham). It seems to me that many men rely on other factors such as wealth and power to attract people, and don't need to give a haypenny-jizz about how they look. This of course is a sad state of affairs.
But it's not absolute, as with pretty much anything else. I myself am VERY conscious about my appearance. Not so much with fashion, but with my weight and body shape. I was always overweight when I was a kid, then lost about 85 pounds in 6 months when I moved to China. I felt unbelievable - sexy girls would look at me and want to talk to me (kind of meaningless really since I'm gay, but...), and I generally just felt fantastic. After that I started to gain weight again, and now I'm almost back to my original weight. Now, I get rejected by handsome guys I like, and my friends always comment on my weight, and it makes me feel so bad about myself.
So there are always gonna be exceptions (like me, for example), but I feel women are generally more concerned about appearance, because frankly society sends that message to them.