agreed with anands mind you equal share is nice too, as long as the CPU isnt some old thing thats always spiking
X guaranteed and Y burst when available is best combination. Host's can do different SLA depending on your requirement.
Actually there are not true answer for it. It mostly depend on the server spec and the client on the server. If you are on a server with client that using too much CPU resources then you will need to chose guaranteed CPU. And vise versa.