Never heard of either, but just used both and the free version of Burp seems complete enough and is free. Charles is restricted when free ($50 license) but looks better.
edit: additionally, Burp uses a custom certificate instead of a default one for all Charles users
Charles is not new, it's been around more than 10 years.
I think that generally saying 'what does X do that Y doesn't' comes across as fairly negative, and unless there's some obvious reason why Y should be the default adds little.
I'd never heard of burp, but I had heard of Charles. Is there some obvious reason why burp should be the default?