Mining is more like proof-of-burn (almost literally!) than spin-off, but there is certainly a similarity. A conventionally mined altcoin can be obtained from Bitcoin via using the Bitcoin to pay for mining right?
I think both proof of burn and straight mining are perfect ways to distribute altcoins as far as fairness goes. Proof of burn is nicer for BTC holders, though, since it increases bitcoin scarcity.
I'm not sure about PoB being "better" for BTC holders. Proof-of-burn forces BTC holders to make an irreversible decision. If you mess that up, you lose out, potentially a lot. A spin-off allows you to do nothing and maintain your ledger position regardless.