but is it better to buy a good head and a decent cab or vice versa ?
Good head with a decent cab is the better option. A new set of speakers a few months on won't cost too much, and you'll end up with a great cab.
if you have to skimp on one, skimp on the cab. but hey, if you are gonna shell out for a nice amp just go into a bit of debt and get a cab that matches quality wise. a good head will always sound good, but a decent head will leave just that bit lacking. a good cab will fill out your sound and make it a touch nicer, which doesnt help if you arent satisfied in the first place. as long as you get a solidly constructed cab you can always upgrade the speakers in it later, so if you must skimp do it there.