Ukraine

It doesn't make much sense to hit the pipelines if the refining infrastructure is broken. In that case they don't have anything to send through the pipelines anymore anyway.

Hitting pipelines is more of a short-term measure to cause pain. They're relatively easy to repair (much easier than advanced refining equipment), and there's often some redundancy so that gas can be shipped through a different part of the network if one pipeline is breached.

On the other hand, the pipelines span such a huge area that they're basically impossible to defend. If you're able to constantly hit pipelines all over the place, you're causing significant strain on enemy logistics. However, as mentioned by another comment, the pipelines are often buried, which makes them practically immune to light drones.

All in all, I think Ukraines strategy of targeting refineries directly is probably better in the long term, even though they're more well defended. It also has the advantage of forcing russia to try to defend the refineries. Even russia understands that defending the pipeline network is basically impossible, so they would rather just take the hits and repair the pipelines, so you wouldn't get the secondary benefit of straining their air defences.