I find the echo dots to have vastly better sync with the multi room audio. I can have 2 of them infront of me and they actually image to between them, they are that locked in sync and its constant rather than the chromecasts which drift apart over time and will have a different offset when you start casting to the group again.
I may have some come up that I will sell if I can find where I put them. I thought they were in with the games console stuff I packed away but I got that out recently and not in there.