don't waste your time w/ USMLErx, Q Bank.
Just stick w/ USMLE world. Study all the answers and you will be very well served.
I took Step 1 and Comlex Level 1 last year and used all three. Overkill? Yes...
I did pretty well on both, but believe that your best use of time and mode of study is U World. It helps you think like the USMLE.