Actually, you can easily look up the specs for almost any servo.
And actually, speed isn't really a function of the type control used in the servo.

For example, if you go to Tower's online catalogue, the specs for those two servos are there.

The two are almost dead equal in specs.
They're both about the same speed, .21 versus .23
They both put out about the same torque, 43 versus 44
And they're both about the same weight, 1.5 versus 1.3

However, they differ SOMEWHAT in price, $35 versus $13.
You can actually buy 3 of the 3004s for $2 more than one of the 3151s.