You guess wrong, it's not as simple as taking LN code, switching a few lines and having a functioning LN on an altcoin. Coins that are closer to Bitcoin in their architecture will have an easier time of pulling that off (but still not easy), but most of the coins that that were released in the last 6 years have very different codebase and architecture, so it would take a ton of effort to implement LN for them.
The answer is that they both have high fees when they hit the limits of their on-chain capacity. So the answer depends on the current state of their mempools. In recent times Ethereum tends to be more expensive.