Admissions isn't my area of expertise, but I'll address what I can.
(1) Have you checked whether any of your target schools has said that it prefers the GMAT over the GRE? (Some schools do; most either don't or don't say anything.) If multiple of your schools state that they prefer the GMAT, then that might sway your decision. For instance, I just checked the Haas (UC-Berkeley) website and they say they prefer the GMAT. I also checked Stanford's site and they say that they have no preference between the GMAT and the GRE.
(2) Are you planning to apply for an internship or post-MBA job with any of the big management consulting or banking firms? These companies tend to ask for GMAT scores (not GMAT or GRE) and we have heard anecdotally that potential candidates have been advised to take the GMAT, not the GRE. Again, something that could sway your decision.
Next, ETS's "official" calculator is not official at all. There aren't any official academic studies that equate the scoring scales of the two tests. It's just an estimate on ETS's part to help people feel more comfortable taking the GRE instead of the GMAT, since the GRE is newer to the b-school industry.
What does that mean? I don't know. It could be that ETS's estimates are over- or under-inflated, or that they are right on. What really matters is how the schools view GRE scores...so there's a little more research to do.
Most schools publish various statistics regarding GMAT scores for their admitted students. Only some have begun to do so for GRE scores. The one place that I've found the most data gathered is on the US News & World Report b-school rankings website - but a subscription is required. When I last checked that site (about a year ago), they had reported GMAT averages for maybe 30% of the schools in the top-20. I would imagine they have more now, and if a specific school doesn't have GRE stats listed, you can check similarly-ranked schools until you find one that does have GRE averages posted. If the schools have similar rankings, assume their numbers are pretty similar, too.
You may also want to talk to some admissions consultants, who specialize in these kinds of questions. MBA Mission offers free 30-minute consults; this would be a great topic to discuss with them. (Yes, any companies that offer a free service are hoping you'll buy something as a result...but you don't have to! And this company isn't doing high pressure sales - I send my own students to them with admissions questions.)
Re: taking the GMAT (my area of expertise!), have you specifically studied / practiced how to handle the adaptive nature of the GMAT and the implications that has for your timing and performance? If not, that may be what's holding you back on quant. Take a look at this:
http://tinyurl.com/executivereasoningand
http://www.beatthegmat.com/mba/2016/02/ ... n-the-gmatand if you're really dedicated, look at the Scoring section of our free e-book The GMAT Uncovered (there's a free copy already in your student center account with us).
If you haven't been approaching the GMAT in that way, then you might be able to improve your quant score just based on that mindset alone. So then maybe it wouldn't be as much of a lift to get yourself to 690+...
Re: the poets and quants article...yes, at first, the GRE was a way for a school to get around lower test scores for an applicant that they wanted to admit. Plus, they didn't know either at first how to equate the two different tests. That doesn't mean that they are less demanding on the GRE for everyone - it was a consequence of the GRE being new (to MBA land), that's all. (And you can see this in the stats over time. As the GRE has moved more to the mainstream for MBA aspirants, scores have been creeping up.) So I wouldn't rely on that. Or, at least, I would ask someone at MBA Mission about it.