These two words have come into being because there is arguably an -ative suffix in addition to an -ive suffix. The reason that -ative is emerging as a suffix unto itself is because of a high occurrence of -ate verbs (differentiate, alleviate, demonstrate), leading to a high co-occurrence of -at-ive words. (This is similar to how the -ical suffix emerged through the glomming together of -ic and -al, and how -ation has emerged from -ate and -ion.)
Thus, we have two fairly similar ways of constructing these types of adjectives from verbs. For many words we just use -ive, and for many -ative words there is also a corresponding -ate verb. But, there are some common -ative adjectives for which there is no intervening -ate verb. OED mentions authoritative and qualitative as examples. There's also argumentative, augmentative, and so on.
The OED says that preventive is the earlier form and preventative is later, but the citations show both of them coming around in the 1600s and first citation of preventative being just 30 years later than preventive. So, agreeing with your etymological sources, both have been around for, more or less, an equally long time.
With this information, it is hard to justify any prescriptive advice that says that only preventive is correct. This is not backed up by current usage, history, or even grammar (unless e.g. authoritative is wrong).
Your World Wide Words link makes an interesting observation; perhaps there is a prosodic influence, where the syllable-stress pattern might encourage -ative, even in certain cases where there is no -ate form. Without doing an in-depth analysis of -ative words, it's tough to know for sure.
There could be regional differences in what is preferred, but I couldn't find any hard evidence one way or the other. (I know I say preventative.)