obviously MAC usersr will be in favor, btw (as you wrote: I dont have to re-buy Photoshop and everything else), you think OS-X comes with photoshop already installled?
else, not every software, unless its from Apple tehmselve, have all the features that it originally (windows) version has. I've seen many features of Office 2003 that arent in MAc version, so is IE (i dont it aymore though).
if u want to go purely for the photoshop and some designing, then go for mac. else i'll prefer windows as though my machine's basic taks is same, but at leat I have flexibility of development environment as well (servers included) whilst, for MAC, you got to buy MAC Server O/S, which is another 250$ or more.