Kyaaa, that's really hard... Ok, here's some pros and cons:
White top:
Pros: It will stand-out more because of the contrast of the sailor part and the shirt being such different colors, more people will probably recognize it right away, it'll probably be hot if the convention is in May.
Cons: It'll stain more easily if you guys are eating! =D
Black top:
Pros: Stains won't show up! XD Aaaand, I personally like black, and like Sakaki-sama said, in Japan, they might still be wearing that one at the time you're at the con ((Though I don't think it should change your decision TOO much, because I don't know how many people will know that/be picky about that for cosplay))
Cons: Didn't you say your pants were blue? Cuz I dunno if it'll match...
LOL, I'm obviously thinking waaay too much into this... Sorry... And you wanna know something funny? I wanted to buy the men's uniform from that same site! XD But it was so expensive..