I would go for the good head. You can pair a good head with just about any cab, be it yours or someone else’s.
In the end you will only sound as good as your playing skill, your bass, your amp, and your cab... but most is in your personal skill. Learning to play aside, I say a good bass is priority #1 (I base this only on desire not logic), then a good amp, then a cab. I love a good cab, and you will eventually want one... but amps and bases have always given me GAS... never a cab really. Maybe that reasoning isn't so scientific. I just see amps as being better investments... people remember amps, not cabs for the most part.
EDIT: get a good head and then get a cheap but good cab like avatar. Lets face it, most everyone uses eminence anyway... and they all have access to the same designs for cabs/speakers. Sure there are some crazy looking boutique cabs that people swear is the best thing since sliced bread... but any solidly built cab will do the job more than well. Usually the performance difference is pretty minimal (and really comes down to preference). Cab design is not a secret science these days. Only the really cheap/crappy cabs will significantly compromise tone.
p.s. I play tube amps so to me the amp is almost as much of my tone as is the bass