
To be honest if it's for pregnancy diagnosis then the image scan, if it's to check for viability (if overdue or problems whelping) then the doppler may be of use to know if pups are alive or dead.
I must say I obviously have become more patient since my early days breeding as I now don't bother with scans, you get what you get, no amount of scanning will make the bitch pregnant, and knowing numbers is pointless as you still won't know how many live pups you will get, and rear.