Powerprep is the old gmat prep software designed by ETS (the previous vendor for GMAT).
GMATPrep is the new gmat software designed by, you guessed it, Pearson vue (the current vendor for GMAT).
In terms of difficulty: GMATPrep is slightly more challenging
In terms of getting used to the user interface: GMATPrep would be the most current since it's designed by the current vendor.
I would suggest you do powerprep to get a feel, and then GMATPrep. And if you're like most people, maybe you want to leave one GMATPrep test alone till the last few days as a final trial.
I have never used the 1000 questions, so I can't comment on that.