Whether speed or accuracy is more important depends on how fast or accurate one might be. One without any part of the other is useless. Also depends on what level one is pitching. At high school or 14-U and maybe 16-U, I'll take the speed, everything else being equal. At 18-U or college, a pitcher really needs movement and accuracy or she'll get lit up, but she better have some speed as well.
As far as what speeds are average at 12, 14 and 16-U, that depends on what type of tournaments we're talking about. Assuming it's a normal Ohio tournament, I'd venture to say that most 12-U pitchers are going to be around 46-52, 14-U at 48-54 and 16-U at 50-56. Of course you'll see slower and faster than those speeds, but I'd guess 75% of the pitchers would fit in those categories.