usmlerx seems easier, but at the same time, they don't seem to test you on the obscure crap that kaplan does. there are some hard questions in usmlerx, though, but they are testing you on things that seem a little more pertinent.
can't say which is better b/c haven't taken the step yet. i would say that a 70% on usmlerx is definitely not the same as a 70% on kaplan. but we have yet to establish exactly what a 70% on qbank means. sure, kaplan will tell you something, but i wouldn't put too much stock in it.