There is not an appreciable difference in question difficulty between the tests. They will all mimic what you can expect to see on test day.
However, since the scoring algorithm of the actual GMAT is a secret, there may be some scoring differences. The 800Score tests tend to score slightly easier on the Verbal side and harder on the quant side. The GMATLife and GMAT Simulator exams tend to score 5% harder on either section. The GMATPrep tests score 5% easier on either section.
Functionally, there are some differences.
800Score tests can be taken individually by section while the other exams have to be taken all together.
You can choose extended time in the options menu for GMATLife and GMATSimulator, while 800score will prompt you to ask if you want to continue after you have run out of time.
The GMATPrep exams do not have detailed solutions for the problems, but you can always email the homework help line if you have questions about how to solve any of the problems there.
I hope this information helps!
Kori & The Veritas Prep Team