Since you're putting up a false dichotomy, I still think having homeless people on the street doing drugs is superior to executing people for having 1 kg of plant material that is legal in a good part of the world.
At least in the US, you could kill as many people as you wanted and it wouldn't solve the problem, you'd just be guilty of committing a holocaust.
Singapore has a lot of issues beyond just their drug laws. Their entire legal system is one of the most broken of any developed nation. Being able to city plan OK doesn't excuse that.