From what I've seen on this site Enthuware appears to have a better reputation and support then Whizlabs at least on this site.
There is at least one Enthuware support on these forums fairly often, not that you can't get support on their web site as you can.
Aside from that there are some other people on this site who are really well versed in Java to help out if you have a question about question from a mock
test in either product.
I've also noticed is that Enthuware costs about $10 USD and Whizlabs cost more.
You may also be interested in know that many people have come to the conclusion that Enthuware mock exams are more difficult then the real exam.
I do not have Whizlabs, but I do have Enthuware and I can tell you that you get seven standard mock tests, plus a foundation test and a final test for a total of nine tests,
With Enthuware it's recommended that you take each test only once and that you wait at least a day between tests.