A brief experience with both Kaplan and UW Qbanks (btw, I didn't take step 1 yet)
Total questions that have been used: ~500 Kaplan, ~1000 UW
Kaplan:
Interface is nice, even it's not similar to NBME's interface.
It opens in a web browser and it's flash based.
Doesn't restrict you use another software during sessions.
You can use keyboard to navigate through questions.
Gives more detailed performance data.
You need to select divisions/subjects each time you want to see how much question left.
Probably due to negative feedbacks, it seems that they added many new questions of which performance data is still in evaluating phase.
UW:
Interface is similar to NBME's interface.
It's a seperate program and java based, it doesn't allow you to acces another program during test session.
It's mentioned that UW monitors your computer during sessions whether you use screen capture program etc.
Automaticly shows number of unused questions.
UW is harder than Kaplan (I score ~8% higher in Kaplan) and contains more 2nd and 3rd order questions. Kaplan would be more useful to make your knowledge base stronger and UW probably would force you to integrate and further associate what you learned.
Overall, I would recommend both of them. If you don't have enough time, UW alone would be appropriate choice.
I'll write more detailed after taking step 1, if I can survive