Gender reveal. It's tacky, but it's the celebration of new life. A mission call celebrates the loss of two years of a young life that could be spent much more usefully otherwise. BTW I've never been to either, but I have attended lots of diaper showers, a much better way to honor and celebrate an expected new arrival.
I really wouldn't want to attend either one. The missionary call reveal is typically less planned. Someone opens up their letter and it's usually over.
From what I've watched on Y/T, some gender reveals have endless games and activities that are beyond stupid.
So far I have yet to watch a missionary calling that involves blindfolded people pinning the tail on the asses (first presidency photo) although that might be an exciting game to play.