OK, here's the gossip: (again, I'm not a scientist, but I'm pretty sure nothing here is grossly misleading).

Some background: M22 is the formula used by Alcor, and VM-1 is the solution used by Cryonics Institute. Both are designed by professional cryobiologists, and M22 is patented (as well as being extensively used for cryoprotection of tissues in 'regular' labs).

There's some info here: http://www.evidencebasedcryonics.org/2008/07/08/vitrification-agents-in-cryonics-m22/ VM1 is extremely stable against ice formation at dry ice temps of -80`C. However it's more toxic, and wasn't designed for anything other than cooling down. M22 is less stable at -80`C, however it's been incredibly cleverly formulated to minimize toxicity, and increase perfusion. It's also got features (such as ice blockers) which come in most handy during the rewarming process, where a lot of damage can occur for cryopreserving organs, etc.

I heard from the designer, Dr Greg Fahy, that he'd run tests holding M22 at -80`C for a week without ice formation, and he gave the impression that too much longer than that might cause trouble.

So as far as long term storage goes, I'd say LN2 is going to be necessary rather than dry ice. However in the future things like Intermediate Temperature Storage (ITS) might make that even more attractive, by preventing fracturing damage.

As a person living in Australia, if I ever died unexpectedly (without having enough time to relocate to Scottsdale), I'd likely be preserved with M22 and sent via dry ice shipping. My take on it is that I'd rather have M22 (Alcor) than VM1 (CI), since good perfusion is so critical in getting a good vitrification in the first place.

Hope this helps!