All truly decentralized projects are considered "decentralized" because no individual (or small group of individuals) has absolute control over the protocol.
The Ethereum foundation could be considered relatively 'centralized' in that users typically trust the foundation's decisions for the protocol. Yes, anyone can make contributions to the source code, but at the end of the day the foundation is making pull request decisions.
Even so, we typically consider Ethereum decentralized because any user (or group of users) can determine that they no longer agree with the foundation's development direction. In this case, a separate group of users can continue development on their version of Ethereum. This is what happened to create the ETH/ETC split. The main point here is that Ethereum is decentralized because individual users have final say about which version of the protocol to follow.
I'm not sure that you can really quantify ETH vs. BTC. The two protocols handle their development process in the same way (a few core development teams work on a few implementations). Debate around Bitcoin has created several strong "factions" that subscribe to different fundamental values for the currency. This inherently leads to the development of similar-but-different implementations. So far, we haven't seen these kinds of frequent fundamental splits in the Ethereum space (except ETC). Does this make either one more or less centralized? I don't know.