I've been using both. People have been saying that the Enthuware exams more closely resemble the level of difficulty of the actual exam while the Whizlabs software is a bit tougher. Personally, I don't think you can go wrong with either one.
I just passed the exam, and I used Whizlabs, but the level of difficulty is far to high compared to the real exam. And the questions of Whizlabs cover more details than the exam objectives require. And then there is their spelling problems.....
I would go for Enthuware if I had to do it again. I tried the free questions and they came much closer to my final score.