We have just been through this very same dilema and didnt decide until the day before my u/s to find out what we were having. That being said Bubs had other ideas and had its legs firmly clamped together so it decided for us that it was going to be a suprise.

I think it really does come down to personal choice, in a way I am glad now that bubs decided otherwise and we can continue on with our little guessing game that we are having. I think you also have decided whether you are going to find out and if you want to share it with everyone around you or keep it a secret because that can also make a bearing on the decision.