Bitcoin Core compares blockchains based on how much work they contain, and not necessarily which one is longest. This is useful for preventing time warp attacks.
This comment on HN says that the electrum client doesn't implement this, and just naively compares chain length:
Yes. It's possible to create a long chain at low difficulty and the original bitcoin client protects against that by computing the total "work" that has gone in a chain. However, Electrum just assumes that the longest chain will be the one which has most "work" in it which is wrong.
It seems surprising that electrum would change a major consensus rule; I can't see any rationale for it.
Does the electrum client decide the best chain on the basis of how much work was done, (like Bitcoin Core) or how long the chain is?