I’ve got Elan’s practice questions and I think they are adequate. Comparatively, the Schweser end of chapter Concept Checkers aren’t very difficult (various questions feel like Level I). I’ve used all three and I think CFAI’s EOC are the best way to go if you could only choose one. Elan’s practice questions are more difficult than Schweser’s concept checkers.
This weekend I did some CFAI Derivatives EOC questions – they are a real workout and took me a long time to complete.
Also, as someone that has used all three, I think there is value in using a diversified group (different sources) of practice questions. I started using Schweser and when I got to Elan and CFAI questions, I saw different types of questions which tripped me up. I know it’s cost prohibitive to use two services, but with Elan you don’t have to buy the entire study package (i.e. you can buy only practice questions).
If you’re limited in terms of time and money, the CFAI EOC questions should be adequate preparation alone. Just my opinion.