VeritasPrepBrian
Good question, and I've been around Veritas Prep long enough I think I have a good answer!
The 7 Veritas Prep tests are the ones we developed while working with the former Chief Psychometrician at GMAT, Dr. Larry Rudner - they use Item Response Theory coded under his watch and have a deep pool of data and questions to draw from when administering an IRT test. I'm biased of course, but they're really good and we continue to add new questions to the mix, use experimental slots to gather IRT data on new questions, etc.
The 5 "Simulator" tests are licensed from a third-party. They're not quite as good, honestly - like a lot of third-party tests they do a pretty good job of adapting and scoring but they're not fully IRT set up and weren't validated and verified by Dr. Rudner. The design with those is to make sure that there are plenty of opportunities for our classroom/tutoring students to practice with GMAT pacing, endurance, etc. At times we've offered as many as 15 tests and there are people who love to grind them out, so we want to give people the option particularly if their studies continue for several months. Our homegrown test pool could probably support 8 or 9 tests but rather than try to stretch our pool of items to cover more tests but potentially sacrifice accuracy of scoring and experience, we just license 5 additional tests so that the "grinders" have the option to take more if they'd like, but our homegrown 7 tests say well within the scoring error margins that Dr. Rudner laid out for us.
So that's the difference. If you're taking 5-7 tests the homegrown Veritas Prep tests are all you need and I wouldn't bother with the Simulators. If you know you're going to be taking 10+ tests we like having the Simulators there to practice the test experience...I wouldn't read too much into fluctuations in scoring, and I'd use Veritas Prep or GMAT Prep as you get closer to test day and want to have an accurate read on that, but the experience of taking the Simulators can be pretty helpful.
i am going to try veritas 7 test