Should I focus more on DS questions rather than on PS?

Interesting question. This is something I haven't thought about too much before. My intuition is to say that studying DS exclusively will help you a little on PS, but not as much as you'd like. That's because on DS problems that appear to involve calculation, the right approach is usually to avoid doing that calculation if at all possible. On DS, there's the advantage that you don't have to know the answer to the 'question', you just have to know whether there is an answer. So a lot of getting good/fast at DS involves spotting opportunities to dodge calculations.

Also, there are certain critical strategies for PS that you can't use on DS. Namely - backsolving (plugging in the answer choices), and choosing smart numbers. They have something in common with case testing on DS, but they aren't the same thing, and you have to practice them on PS problems to get good at them.

In short:
- study both DS and PS.
- on the DS side, the reason is that you should learn to avoid calculation. If you want to improve your PS, you sometimes do have to do calculations.
- on the PS side, the reason is that you need to learn two critical strategies (at least) that don't work on DS.