Was going to try and provide an answer but lukeprog's link is more informative than what I would have written :P
One factor though that to me seems worth considering:
Imagine your goal of people being able to back themselves up turns out to never actually happen - either because of technical infeasibility, or we go extinct before figuring out how, or because static structural information is simply insufficient to derive psychological phenomena. (That last option might be a minority position on LW, but I'd venture to say it is not a minority position among neuroscientists and psychologists.)
If that WERE the case, then money and time donated to medial imaging is likely to STILL have enormous positive benefits, in terms of advancing basic science and diagnosis/treatment of mental illness. By contrast, if your end goal turns out to be infeasible, money and time spent on cryonics and plastination will turn out to have been largely wasted.
Or at the very least, it is much easier to imagine the former having a large societal benefit for reasons other than life extension, while the "side-effect" discoveries of brain plastination would not have the same obvious public benefit (even if those benefits were non-zero).
So it seems like if you're not sure which is the best approach and want to be fairly certain you're not wasting your time/money, it's better to dump resources into medical imaging, which would be justified by the spillover effects alone.
I thought CLARITY was an interesting development - a brain preservation technique that renders tissue transparent. I imagine in the near future there's likely to be benefits going both was from preservation and imaging research.
To all my physicists in the house, will it ever be possible for a device to scan the contents of a human head at the molecular level (say, 5 x 5 x 5nm) while the subject is still alive? I don't have a physics background, so if you could also just point me to the materials I need to read to be able to answer the question, that would be wonderful as well.
The background: I want to live to see the far future and so I'm researching the feasibility of alternatives to cryonics that'll let people "back up" themselves at regular intervals rather than at the point of death. If this is even theoretically possible then I can direct my time and donations towards medical imaging researchers. If not then I'll continue to support cryonics and plastination research.
I'm looking forward to your responses!