Probably not the answer you want, but it depends. Which conference? Which Journal? In general a journal is better, but a paper at super hard to get into conference A is better than a paper in ridiculously low level journal B. At least in my field anyway. At my uni we are encouraged to publish, usually with the more preliminary stuff in a conference and with the full work in a good journal. I see it as - each publication is most of one chapter for my thesis. I intend to have either 3 or 4 publications by the time I finish (I have one and am writing up another and am 1.5 years in). Probably two conference and 2 journal if possible.
The paper I've written so far was a conference paper, but at a prestigious conference, and is viewed as fairly good. The one I'm writing is for a decent (but not the highest level) journal and would be viewed as slightly better. It is a continuation of the work from the conference, but much broader in scope and more detailed (with a lot of extra unpublished work added!).
Obviously this is just my experience doing an engineering PhD whilst working as an RA at a certain uni. Perhaps it's different elsewhere and in different fields?