Wishbone
I think a lot of the Quant is spot on, but the verbal tends to vary a bit more, just my opinion, but you can find oodles of posts on this topic all over the site, maybe a small search could help you out
I
love Wishbone. My other favorite Jack Russell of celebrity is Rimshot.
And I would say that Rimshot and Wishbone are more similar than
MGMAT and GMATPrep, but I'm not sure who is which.
MGMAT has harder questions, yes, but I think your comparative results may have a lot to do with your style of test-taking, especially your willingness to let go of a problem and move on with a guess.
VS 
Let me explain - assume two 700 range test takers, a stubborn one named Rimshot and a practical one named Wishbone. Rimshot is going to get killed on the
MGMAT CATs because he sticks every question out to the very end and wastes time (on a lot of them, because he's getting them right), paying a heavy penalty at the end of the test when he doesn't finish. He finishes the GMATPrep tests easily, though, and scores well. The end of his
MGMAT scorecard looks like this (percentiles):
... 99, 99, 99, 99, 99, 80, 73 (finishing weak due to time)
Wishbone, the practical one, gets more answers wrong on
MGMAT because he moves on very strictly after 2 mins. His
MGMAT scorecard looks somewhat like this:
55, 65, 80, 91, 99, 88, 90, 99, 88, 75, 91, 99 (getting the last few right puts him well into 700 range)
Wishbone gets more questions wrong, but leaves himself a shot at finishing strong. Rimshot has more right answers, but gets to answer fewer of them in Ernest. Guess which one I was.
Because the GMATPrep questions are easier, Rimshot has a fighting chance to finish all of them, and Wishbone doesn't need 2 mins on each problem. They both score well b/c neither is pressed for time, locally or globally.