Hi Khulan,
Assuming that you took this practice GMAT CAT in a realistic fashion that matches-up with how you would have to take the Official GMAT - and that this Score is a reasonably accurate assessment of your skills - it certainly appears that your GRE studies have helped you to hone some of your GMAT skills. Considering that you're coming off of several months of GRE study (and know more about the overall format and content of that Exam), I think that you would probably have a better chance at hitting your GRE Score Goal (in a shorter period of time) than you would hitting 700+ on the GMAT. Thankfully, both Exams are consistent and predictable, so you CAN train to score at a higher level.
Since you're not facing any immediate application deadlines and you appear to have plenty of potential study time going forward, you can approach the next phase of your studies (for either Exam) however you like. Are you more 'comfortable' with either of the two Tests?
GMAT assassins aren't born, they're made,
Rich