With regards to Virtual Appliance vs Physical Appliance, our engagement with customers shows that it is much easier to go with the hardware appliance in the beginning. When partners become experienced then virtual may be the way forward for them. We would love for all our customers to be virtual or software only but the reality is, being a core part of the network, that the appliance is much easier to deploy. So it is highly recommend going for the hardware probe in the beginning.
You can also check out this short video that outlines the differences between Physical and Virtual Probe.