I would say wait for the 3D till at least 24 weeks. I just went for one at a specialty place in Denver, I recommend them. They're called First View Ultrasound. I am probably going again in a couple of months for the 3D.
They say to wait for 24 weeks so it looks more like a baby. There are pros to waiting longer too.
At 24 weeks you get more movement and then closer to 30 the baby looks more like a baby and is more filled in and as the U/S tech said.... "cuter" lol.
Are you just waiting to go at 20 weeks? The place I went can usually determine as early as 13. :-)