It shouldn’t be an either or question. You can do both.
Muscle can be built through a wide variety of rep ranges, whereas strength tends to respond better to higher weight, lower repetition work.
Whatever you do- low reps or high reps- you must try to improve. This could mean picking the same weight and doing more repetitions each session. It could mean using the same repetitions and adding a little more weight.
There are lots of options, but the underlying principle is progress. This is why a training journal is critical to long-term success. You can’t remember everything you do- write it down in a notebook, and then improve something each time you train.
