For most standard farm ponds — water storage for irrigation, general livestock water access, rainwater harvesting — the 400 micron liner is usually the right call without needing to think too hard about it. It's meaningfully more resistant to punctures and general wear than the 300 micron option, which matters for a pond that's expected to stay in service across multiple seasons rather than being seasonal or temporary, but it doesn't carry the extra material cost of the 500 micron tier that's really built for the most demanding, permanent installations.
If you're not sure which thickness your project needs, this is usually the safe default: durable enough for genuine multi-season farm use, without over-specifying (and over-paying) for a level of puncture resistance and lifespan that a standard farm pond doesn't actually require.