VARs should recommend which ever solution the customer would have the easiest time managing. If the customer has no Linux experience at all then Hyper-V would be a better option. If the customer has some Linux knowledge then VMware may be better as VMware has the same look and feel of Linux.
From what I’ve seen of both products the VMware ESX is a much thinner host OS, taking much fewer resources. And with the new features which are in ESX 3.5 such as Storage VMotion, ESX appears to be a much stronger contender.
VMware also has auto load balancing via its VMotion system, as well as data center recovery solutions which as far as I know Microsoft doesn’t have.