Neither.
I usually do a bit of both. It is important to practice slowly so that you can get the feel of the notes. However, it is also important to practice quickly. If you just increase in speed slowly, you will encounter a speed wall that you will have trouble getting pass.
The trick is generally to occasionally practice above performance speed, but hands separate. Once you do that, you will find that the slower speed is much easier, and you can even play it a bit faster without losing accuracy.
However, don't practice the whole piece like this, but extract a measure or two, or even a beat or two, and practice those repeatedly using the above method. If you like, you can practice a section of the piece around those troublesome parts, or the whole thing if it is not long, but at a slower speed (from 1/2 to 3/4 speed). The last time you practice it should be slower.
Also don't practice these parts for too long. Maybe do 10 minutes or so for a troublesome part, and then move onto something else.