Hi macgyver - BB is my favorite of the 3 ES siblings I have ... a reliable bloomer, as upright as Forever & Ever due to thicker canes & the gradual color changes from mid-Aug into Fall is absolutely awesome ... final stage is deep burgundy!!!

I don't own MEM to judge & compare.

'Mme. Emile Mouillere' may very well be root hardy to zone 5 but is highly unlikely to be bud hardy in that zone without winter protection. It is one of my favorite selections and a very heavily blooming white mop that developes a rather distinctive bright blue eye. It may be worth going through the effort :-)

Identical by hardiness to MEM, Sister Teresa is NOT a reliable bloomer in 6b. I highly doubt that MEM will be blooming in z5 without serious protection.
I guess that would leave Blushing Bride as the only choice for white mac for z5.

Thanks for the info. I guess I'll go with BB to stay on the safe side. I am also trying a couple of serratas recommended here for better hardiness. We'll see next spring! I moved from 6b last summer and still can't accept that 60 miles difference significantly limits my hydrangea selection. Thank goodness for the rebloomers and the paniculatas.