Supposing you're playing with /mixed/ default decks and some phantoms only, I think best option would be to buy few cheap cards, but which will improve your deck a lot, as this can be easily done. Will help to get another 150 gold way more quickly..
Buying some booster would be mostly only waste of the hardly obtained gold, as there is much more not-so-handy rares than those, which you would sell easily. Of course, there's always chance for those, but you need to be really lucky. Buying single needed cards will be safer..
Buying booster as soon as I collected my first 150 gold was my biggest mistake.. so you can believe me as I did this foolish thing =) (But maybe it's because I am not the lucky one, you might be, if you feel courageous enough to do it
)
So I purchased pack! But I was quite lucky and i got some expensive cards. 
