The main difference is that bitcoin is completely decentralized while ethereum isn't.
Very true and main reason of it boils down to PoW vs. PoS. Quite contrary to PoS, PoW is very hard or almost impossible to compromise. Not so for PoS, where more attack vectors can be found.
For example, PoS can be abused by rich stakers. Many PoS projects have a so-called pre-mining of coins allocated to the team and the team can abuse such pre-mined coins to gain influence about project decisions.
A large pre-mine is making a project centralized in many ways and devs can gain much influence by staking a large amount of PoS coins. PoW is not having such issues.
A similar problem is called “nothing at stake”, where attackers benefit from no cost to stake ETH or similar PoS coins. Should a fork occur, no matter if the fork is a friendly, accidental or a malicious attempt to rewrite history and reverse transactions, it’s a given strategy for any staker to stake on every chain. By doing so, he will get a reward no matter which fork will succeed and he will have no additional cost to do so.
In PoW, such a problem is not happening because PoW always means work (spending electricity) needs to be done. A cost occurs for every miner.
