Bitcoin Unlimited is at the moment the dominant version, as measured by mined blocks: https://coin.dance/blocks

The vast majority are picking a side for political reasons - "Core" is embracing censorship as method to push their client, Unlimited believes in open discussion.

This is best exemplified on Reddit, where /r/bitcoin is moderated, heavily, by core, but /r/btc has a no-censorship policy.

This makes the technical differences between the two almost irrelevant, most are picking a side because they don't (or do) believe in censorship.

"Bitcoin" is normally defined as whatever is the longest continuous chain of blocks, which if the current state of affairs continues, will be Bitcoin Unlimited. But somehow Core have managed to stake a claim to the name "Bitcoin", which is going to cause massive confusion if a fork happens.