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For collecting, 1:12 is what I like. You get a good mix of price, detail, and quality. If you're buying western toys, it usually leads to a lot of reused molds, so you end up with a feedback loop of buying basically the same figure over and over again, while eastern figures seem to get stuck with endless delays, and they also reuse molds again.
If you're actually playing with your toys, then 3.75" is better. Smaller figure means more options for playsets, vehicles, accessories, and obviously figures. Not only that, but they're cheaper as well, so army building is much more affordable.