imo, both r equally impt. We all know how impt the head is, but cabs r just as impt in my opinion.
The size of the cab need to be calculated carefully, and built sturdy. U wont want some cab that buzz or rattle when u crank rite? Also this is where the "sound" get much resonating. Also, the speakers r designed with dif freq characteristic so that also affects the tone. N other things like the air its pushing. 2X12 open back and 4X12 close back also does make a dif...
Also a excellent overdrive not not just the cranking preamp n power tubes, but as well as how the speakers break up...
So a gd head thro a lousy cab will sound not v nice. I heard muddy cabs b4, n didnt like them. Ever seen reviews where one guy badly rates the amp, but praises it when he fit in some jensen or celestions?
But if cost is a factor, than its better to settle for a better head (One that u plan to hold for long), n get a suitable cab. Its more practical n logical to upgrade the cab than head