I think translational is sort of the sweet spot for med students, because you'll work with both basic scientists and clinicians and also work with projects that are usually mature enough to publish on. It's also very field dependent.
Neurosurgery/neuroscience tends to be very basic science philic, along with the other surgical fields to a somewhat lesser degree. I think ortho and to a lesser degree plastics value clinical research a bit more.
IM can go either way but some of the subspecialties are super basic science oriented.
In my field of interest, rads, all kinds of research would be useful, but basic science/translational carries a little more cachet.
Ultimately you can't go wrong with basic science for most disciplines, even if they aren't necessary relevant to what you end up going into, but it's very easy to work on a basic science project for a long time without publishing.
As far as how much to do, Ive spent 10-30 hours a week in lab during my preclinical years, which is on the high side, but gotten nice results from it (8 accepted abstracts, 4 accepted pubs with 4 pending) You can do as much research as you want in med school, it's just all about balancing your time.