Mine was external. It takes longer because they measure everything, head, limbs, organs. They make sure everything is growing as it should. The last thing they did was the gender check. But it was cool to see the baby so the wait wasn't bad!
Finding out the gender is really just a bonus. The goal of the 20 week u/s is to examine the baby's systems and organs to make sure everything it developing properly. It can take quite a while, especially if the baby is in an awkward position. I had to roll onto my side two or three times so the tech could get good shots of the heart.
Re: Gender Scan question
DD#1 born June '09
DD#2 born April '11
TTC #3 as of July '14