Your quant dropped a little bit, but your verbal is really the big difference. How was your timing on each test? Did you run out of time and have to rush? If so, on how many questions? Give us as much detail as possible on the timing.
Did you take under exact testing conditions? (essays, 10 min break, quant, 10 min break, verbal) Did you take each at the same time of day? Did you use the pause button at all on the MGMAT test?
Which did you take first? And how much later did you take the second? (If you took GMATPrep second and close in time, such as the next day, you may have just been mentally exhausted by then.)
Were there any differences at all that you noted between the two tests - your energy level, your timing / pacing, how you felt during the tests, how easy or hard you felt the questions were, etc?
I do like our own RC and CR books, but then again, I'm biased because I work for the company! If you have friends who are studying, you may want to ask them what they like. You may also want to ask the opinions of other students at
http://www.beatthegmat.com (lots of experts from different companies answer questions there, but the site itself is not affiliated with any one company).