For me it varies.
I like the look of a head and cab way better.
I like the portability of a head and cab way better.
I like the simplicity of a combo when I am not going to be moving it.
I like the one trip to the vehicle and it is loaded action of the combo.
I like the sound of the open back of the combo mixed with a sealed cabinet and/or one or the other by itself.
I like that tubes tend to last a little longer in a head v. a combo.
To me it doesn't matter what the amp is housed in provided I am getting good tone.
I often disconnect the combo speaker and run a Thiele with a sealed 4x12 cabinet when in the combo configuration.
I own all three configurations for the short chassis (head, combo, and rackmount) so I can change between the three if I so desire though I have yet to rack it because I haven't felt the desire to as of yet.
I am fortunate to have owned the EVM-12L in the combo so that I may be pumped up. I make all you girly men wish you had a physique like mine... Truth be told, The combo with the EVM is about as heavy as a long 2x12 combo sporting other brands of speakers (Celestion, Emminence, what have you).
I like the fact that the head sits lower and can hide under a blanket in the extracab seat of my truck or sitting front to back the longway on the front seat between the driver and passenger like an armrest.
I like the fact that since my combo cabinet has wheels it rolls where I want it to.
I like the big feet on the head cabinet but do not like the black circles that they can create.
I probably would have would have voted for the head due to reduced tube life in the combo and the stylish looks sitting atop my 4x12 cabinet. The combo's open back setup is something I can work around because my Thiele is actually tuneable with it's back door feature allowing you to choose the amount of openbackedness ( <------ just made that word up do you like it?) and it has an additional panel that you can tune the front ports with also.