I have used mock exam softwares of both Enthuware and Whizlabs for OCJP 6 very recently and the following has been my personal experience:
1) High on quality and quantity, value for money.
2) Even after one is finished with the objective wise Qs, very few Qs repeat (if any) in the complete mock Tests.
3) Very few Qs on "out of syllabus" APIs.
4) Software can be installed on any of your systems and you do not need to be online.
1) Low on quantity. Not value for money if you compare with Enthuware. See next point for more.
2) I started with the objective wise Qs and then moved to the complete tests. There were hardly any new Qs. Most of the Qs were repeating. Their customer care executive (Raj) acknowledged this fact!
3) Quite a few Qs on "out of syllabus" APIs.
4) You need to be online and login to their website in order to use the software.
Summary: As per my personal experience, Enthuware is and Whizlabs is