The main problem I'd think is that by being "virtual" you never really quite know if you're seeing how it would run in "real life".

Personally, if I were to do it, and I had some sort of load-balanced application that uses multiple backends/frontends/whatever, is balance across the two and compare over time.

And to be 'fair' you'd want to compare similar pricing.

Or if you had a consistent group maybe try a Minecraft server and move it between the two every week and see what people "feel"?