well at the end of the day the decision is up to you both & it's your special day so you do whatever you want that makes you both happy . i personally don't really like the idea of surprise weddings. like Nel above said you will get people who won't show, but if they knew it was a wedding & lets say they "felt sick" they would make that extra effort to come because it's your wedding, not just a party IYKWIM. also you could get people coming in very casual clothes, that might not bother you, but it would bother me. there's more reasons but i just can't think for now.

all the best in whatever you decide to do