You are right, there is no diffrence except clock speed. They both use the Manchester core so you would be better off getting the 3800 X2 and overclocking it to 4200 speeds and saving money that way instead of paying for the same chip that comes clocked faster.
The other X2 core is the Toledo. The diffrence is the Manchester has a 512kb L2 cache and the Toledo has a 1Mb L2 cache.
Many people like to say the Mancherster core is like 2 Venice cores, and the Toledo core is like 2 San Diego cores, which is pretty much true.
Im gettin the 4400 X2 soon, cant wait!!!