Circling back to update:
After much deliberation I've put my money down on the m500.
While there remains precious little in the way of objective comparisons, lab tests, or any other material that doesn't amount to a marketing/PR
release for the R400 family, I think the m500 is the right call.
First, educating myself further on the topic, I've come to learn stats like Mbps are meaningless in comparison between two different cameras, for EXACTLY
the same reason megapixels are meaningless in comparison for still cameras. Higher Mbps for any given compression method has no direct bearing on the quality of the final product. The quality of the hardware (lensing, sensors, processors) have a much more direct impact.
On sensors: Since the R400 family uses the same sensors as the previous iteration (R300 family), we can make the assumption the R400 will have similar pitfalls to the R300 series relative to the M500 in low light scenarios. There are multiple accounts (on amazon among other places) of folks trying the R300 series only to be unhappy and upgrade to the M500 series due to image quality issues.
On 1080p: I think this stat is the only legitimate reason one would opt for the r400 (besides price), but I think most folks (myself included) would prefer a 720p image with better quality/clarity to a 1080p video that in comparison looks grainier/fuzzier. Further, for situations where interlacing isn't such a big deal (I'll have to learn where the line is drawn), the actual end-user footage of the m500 available on youtube/vimeo, per SD90's suggestion (thanks!), really helps seal the deal - folks using the m500 with either 720p or 1080i are achieving STELLAR results. This tells me the camera can do what I'm looking for, I just need to develop the skill sets to use it properly!