The problem with pay-as-you-go is that there is a financial incentive, when faced with something that you're pretty sure about, to say "nah ... I won't pay to check the record that will confirm it ... what I've got is good enough"; and then you can be led astray if it turns out to be wrong.
With a subscription, the more things you look up, the better value you're getting for your money ... so there's a financial incentive to check, check and double check, and always to look at that record just to be sure.
So I would say subscription is better, if only because it incentivises and therefore encourages good research habits rather than poor ones.