I disagree, it really doesn't matter if it's a tank/mage/assassing/peeler for me. Generally, you just need to evaluate the kill potential on both and determine which kill would be more secured.
For example, if the support flashed and exhausted and the ADC healed, I'd go for the support. Chances are if you go for the ADC, he will burn flash but survive but if you go for the support, you can kill him, and basically zone the ADC a few waves until the support returns to lane.
Or if ADC burnt Flash and Heal, but support still have both, go for the ADC.
Always be aware of mobility. For example, if it's an EZ/Cait/TristADC and a raka/taric/thresh/blitz support, I'd always go for the support as they don't have a skill that could allow them to escape but their ADC do. Vice versa, if the enemy ADC is Varus/Kog/Mf and support is Braum, I'd go for the ADC since Braum can jump on to the ADC for safety.