I guess I'm not a great fan of using a mems + cpu based unit as a backup.
I like my backup to fail for a different reason than the primary failed.

Aside from that, the unit-volume of all aviation digital ahrs systems
combined
is so minuscule that if you told me no one had ever been seen to fail in the
air
I still would not feel warm and fuzzy.

if you are just sold on digital ai's I would find out from Chelton and if
possible
pinpoint, just how much testing it takes to get these things to work most of
the
time and then see where these backup units you mention stack up.  I would
expect
there is a huge gulf between them, and even then, Cheltons have been known
to
fail.

I went with the full size battery backed life-saver gyro from mid-continent
as my primary and
a standard vac ai as my backup.

Colyn