I have 8GB RAM in my Macbook Pro and usually run Windows 8.1 in a VM all the time, and have assigned 4GB to each (sometimes I only assign 3GB to Guest).
I often have quite a few apps open in both host and guest, including Firefox with a lot of tabs. After a while switching between host and guest doing various things, I find the guest OS is relatively sluggish, most likely due to VM memory being swapped out to disk (I can hear a lot of disk access). The slowdown seems worse on the guest, but is also noticeable on the host sometimes.
According to the Fusion/Parallels comparison, on Wikipedia, Fusion uses active memory for hosted/guest OS's, while Parallels uses wired memory for hosted/guest OS's. What this means is that the memory assigned to a Fusion VM can be swapped out to disk if the host (ie. OS X) needs more RAM, but the memory assigned to a Parallels VM CANNOT be swapped out to disk when the host needs more RAM.
Parallels has an advantage in this respect, in that the guest VM will continue to run at a faster speed when the host is short of memory, whereas a Fusion VM will slowdown. The disadvantage of this is that the amount of RAM allocated to a VM is lost while that VM is running (and probably paused).
Given my use-case, I personally prefer Parallels choice of using wired memory for VM's, as it would kep my VM running faster.
I can see a need for both options, so can you add a per-VM option in Fusion to allow the user to choose whether the memory assigned to a VM is wired or active. Unless there is already a hidden option to do this ?