i use both
i have a 4 node ESX cluster and am building a 2 node Hyper V cluster for our local test setup. Hyper V looks pretty good and is easy to use, but it has nowhere near the configuration options when compared to ESX/Vsphere.
another annoying thing about Hyper V is that the virtual SCSI controller it uses, is not recognized by FreeNAS, which i use for all my datastores and cluster storage, forcing me to use virtual IDE controllers, with all their limitations. ESX/Vsphere has no such troubles. of course i shouldn't complain about this really, as FreeNAS uses FreeBSD as host OS, it's no surprise that the Microsoft virtual SCSI driver wont work on that.