What's a better workout? Low weight many reps or heavy weight few reps?

I want to workout again but I don't know what I should do. I want to become stronger (slightly visibly more muscled) and maybe loose some weight.
I went to a small gym nearby half a year ago and they only had group lessons with light weight and many reps. Like 2 minutes long squatting and then switch to another muscle group or cardio. I saw not a lot of results and money was also an issue so I quit.
However, I go to the gym sometimes with my boyfriend and he does a lot of heavy weighting with not many reps and many sets (building up weight). Like deadlifting, squatting with a bar, bench pressing etc. I do see results very quickly from this type of workout.
So what's really better? I don't want to be a bodybuilder so if I would do heavy weighting then I shouldn't do it too often.

(I have astma by the way and so I don't really like cardio. It's possible, but not too much)
What's a better workout? Low weight many reps or heavy weight few reps?
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