Regarding traffic..
BTW, in terms of traffic, WinCustomzie gets vastly more traffic than ThemeXP does. The only site that has themes on it that gets more traffic than WinCustomize is deviantART which is more of an art site. Statistically, there are no "skin" sites that get the traffic of WinCustomize on a daily basis. You can use tols like Alexa.com to view relative traffic rankings.
ThemeXP is a good site, I visit there a lot for wallpapers. But presently, msstyles are not nearly as popular as WindowBlinds skins both in terms of daily downloads as well as new skins created. Using the way back machine...
Regarding which skin format is more popular today...
ThemeXP has about 900 msstyles now. Using the wayback machine in November 2001 they had 66. So roughly an 800 skin increase in 20 months (40 per month).
At the same time, WC had 1483 in January of 2002 and 2314 today about 900 skin difference in only 18 months (about 50 per month).
Plus there's the whole "Look, it's Luna but now it's navy blue" that so many msstyles have. There are some excellent msstyles out there but there's a lot less variety available and fewer skins made. And since WB is a superset of msstyles, good msstyles can be brought over to WB quickly. Msstyles has some outstanding authors such as Kol and Chaninja and some of Lightstars are good. They've done a lot with that format.
Regarding which gives the skin author more exposure...
But if the argument is which format gives the skinner better exposure, it's a no brainer- WindowBlinds. When Microsoft wanted to promote the Xbox, it wasn't msstyles they used - it was WindowBlinds. When Warner Bros. wanted to promote Terminator 3, they used WindowBlinds. When Microsoft wanted to promote Rise of Nations, they again chose WindowBlinds. And when they choose WindowBlinds, they pay authors to make the skins. Pixtudio, Skinplant, The Skins Factory, Essorant, MikeB, and others have all seen their work put in front of many millions of people. No msstyle skin author can say the same.