When we were developing Hyper-V in Windows Server 2012 R2 we did a lot of testing to try and come up with an answer to this question – and the answer is surprisingly simple. All you need to know is: how much network bandwidth do you have available for live migration?
If it is 10gbit or less – use live migration with compression.
If it is greater than 10gbit – use live migration with RDMA.
It does not matter if you are using 1gbit, 10gbit or 40gbit network connectivity – this guidance still holds. It actually does not matter if you have RDMA support or not. If you have over 10gbit network connectivity available for live migration – live migration over RDMA will get better performance thanks to SMB multichannel support.