It depends on how do you compare them and what do you want to use them for. Ethereum is better than Bitcoin and Bitcoin is better than Ethereum at the same time.
They’ve made several completely different design choices and as with every choice it opens some oportunities and closes dosens of others.
I would highlight a couple of main differences:
Purpose
Bitcoin is a currency
Ethereum is a platform for running decentralized apps (aka smart contracts)
Supply
Bitcoin is deflational
The overall amount of BTC ever produced is limited
Ethereum is inflational
The ETH supply in unlimited
Security
Bitcoin is simple and limited
The built-in language is stask-based and stateless
That’s why you can not express a loop and it’s not Turing-complete (this means, that there are some programs, that are impossible to write)
Ethereum has a rich programming language built-in
The built-in language is Turing-complete (this means that you can code anything you want)
More complexity implies more opportunities to make an error
Adoption
Bitcoin has a first move advantage
My mom knows what Bitcoin is
The community is a lot bigger
It’s easier to get started
There are already a lot of places where you can pay with BTC
Ethereum is less known and younger
It uses “move fast and break things” motto
It learned from Bitcoin and fixed a couple of issues in it’s design
The community is smaller and therefore has more developers than speculators
To sum up:
They’re not competitors
They coexist and solve different types of problems