It took me a fair bit of Web searching to find the answer.
In the end, I finally hit upon the correct set of terms to enter into Google. I did a Google search for [
plantronics m70 m90 vs | comparison | difference ]. The 21st search result was an Amazon.co.uk product-description page which included the information I sought.
On that page, reviewer Andrew Oakley writes:
Note that the only difference between the M90 and the M70 is that the M90 comes with a fancy charger cable that can charge two devices (headset & phone) at a time, wheras the M70 just comes with a standard charger cable. The M70 is mostly black with a thin white stripe along the edge; the M90 is entirely black. The M70 and M90 headsets are otherwise identical in all technical and physical respects, to the best of my research. Talk time, A2DP, BT3.0, spoken features, etc.: all identical.
An anonymous commenter adds:
Plantronics don't make it very clear about the difference between the two products. I'm certainly going to go for the M70. I even checked the manual on the Plantronics website. It groups the M25 & M55 in the same manual and mentions when there's a difference. The M70 & M90 have the same manual but the only difference seems to be this for the "What's in the box" section: *Contents may vary by product.