I would rather know but not tell due to the gendered BS, but we made the mistake of telling MIL we would probably find out, and now she is acting like a freaking pitbull. She got out of us when our appointment was (this Friday), so now if we decide not to tell, it's going to be a huge thing. I really don't understand why this is so important to her. She already has 7 grandkids, 3 boys and 4 girls. She is also prone to things like commenting to DH's cousin that her 1-year-old was "all boy," and going on and on about how it's good for girls to be "petite." I am not looking forward to this.
I guess the bottom line is that apparently I am weak and care too much what MIL thinks, or we would just say forget it and not tell her... 
ETA: In response to the original question (which I was considering before getting sidetracked on my rant, LOL), I guess I never really struggled with the decision... not really. In an ideal world I think it would be best not to know, but I know I am too weak (this seems to be a theme with me) to resist finding out when the ultrasound is right there in front of me. DH also wants to know, so he'd be no help.