UWorld does not, Kaplan does, and USMLERx isnt worth your time
As far as I could tell (i used both World and Kaplan for Step 1) the only advantage Kaplan has is the Re-Kaps and references to First Aid (And MedEssentials).
In general, the style of questions, difficulty of questions, and even the very interface is most like the real deal with UWorld. If you are training for the test, train like athletes do for the big game. You do the thing most like the competitive event. Thusly, use UWorld.
Kaplan is more difficult, requires more detailed knowledge to get their questions right, but has a heap more information when it comes to the content. If you were going to use a Qbank to learn (as opposed to reading First Aid or other review books), Kaplan is better.
If you are going to choose one for Step 1, pick Uworld.
If you are going to prepare hardcore, do Kaplan FIRST during your second year or towards the learning portion of Step Studying, then switch to UWorld to train for the big day.
If one Qbank use UWorld.
If two Qbanks use Kaplan First, then UWorld
Kaplan has reKaps and references, but UWorldhas interface and question quality.
For Step 2 the answer is obvious, UWorld the whole way