I have asked this question before
Can we use "better" in this way?
'I like Science better then Maths.'
or do we have to use "more"
'I like Science more than Maths.'
The answer I got was: you should use more... but is 'better' usable in an informal way?
No, you should also not use better in an informal speech. It is non-standard English and also not correct.