Now it's jumping the gun for me to say this since I have yet to have seen the USMLE or COMLEX-USA, but I wouldn't be so sure about the COMLEX being easier than the USMLE.
But from what I can gather it goes like this:
The USMLE, has far harder molecular genetics and medical biochemistry and tons of health statistical information.
Wheras the COMLEX had much harder pharmacology and anatomy related question - and of course our most beloved OPP section.
So, to the original poster, if you are strong in molecular genetics and biochemistry - then finnish the USMLE series and don't finish the COMLEX-USA series. If you are strong in pharmacology and anatomy, then stick with the COMLEX-USA and don't bother taking USMLE.
Hope this helps.
C&C