I think Kim highlighted a good point - if you're really hoping for one or the other, you may be better to be prepared - and if that is the case whilst i don't say 'push your DH' try and explain to him all the things that may occur if it's not the gender you wanted at birth - difficulty bonding, pathway to PND etc.

Otherwise, if you're not too fussed, I think leave it a surprise, gives you something to be excited about when you're going through labour all over again.

ETA: you beat me to it when you posted!