You may well already have a preference as to whether you'd like a boy or girl, as may your partner. So prepare for the disappointment if it's not what you'd imagined. Whereas waiting for the day of the birth, you'll be so shit scared that everything is going to go ok, and be so overcome with emotion when it comes out that you really won't care.
For us, there are so few surprises left in life, that I was happy for nature to throw one our way. As for the acquaintances I have that knew/didn't know, there was no way of predicting who wanted to or didn't want to - I have been constantly surprised by who has or hasn't found out. So, I'd try not to judge anyone whatever their decisions. It's very personal.
In your situation, if your partner wants to keep it as a surprise, then I'd go with that. There are many decisions to be made and most of them are the same whether it's a boy or girl and the most important thing is that you're both on the same page when the decision has to be taken.
Best of luck with the scan though. The 20 week one is amazing. I think you'll be amazed at the level of detail that the newer machines can show now. And remember to tell the ultrasound person beforehand whether you want to know. That way, he or she can tell you to look away if there's a point at which you might find out.