What Is the Main Security Trade-off between PoW and PoS?

PoW's security relies on the immense economic cost of energy and hardware required for a 51% attack, making it highly secure against external attack. PoS's security relies on the economic cost of acquiring and staking 51% of the network's token supply, and the threat of 'slashing' (losing staked funds) for malicious behavior.

The trade-off is energy efficiency (PoS) versus battle-tested security (PoW).