Which type of paper is best for computer science, journal or conference?
Computer science is a bit of a special case, in that high-profile conferences can be even better than high-profile journals.
Here's an in-depth discussion.

But it's still important to get journal publications because the rest of the scientific community (including perhaps the chair of your department) tends to believe they're better.

Now this distinction is only important once you have a few publications already. 

Also, one of the advantages of conferences is that they have set deadlines and processing times, constrained by the conference dates, whereas journal submissions can take months and months to be processed.