Effective strength training is all about genuinely challenging your muscles at close to their maximum capacity.
This means a weight heavy enough to fatigue your muscles in 12 reps or less (and fatigue means you feel you cannot do another rep with the correct form - the harder and fewer reps it takes, the more effective the ST is. So it sounds like you should go up to a heavier weight. But you should only go up the minimum increment possible - probably 2.5 or 5 lbs. If you can still do 12+ reps at the new weight, go up another increment next time.
You get about 70% of the maximum possible advantage with the 1st set, 90% with the 2nd set, 95% with the 3rd set, etc. There are definite diminishing returns to adding more sets.
If you have to choose one or the other - go heavy, rather than going more. But I'm not sure why you have to choose - heavier weights will mean fewer reps (and less time) in each set. So you can complete more sets in the same amount of time.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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