What is the difference between float() and toFloat() in GEE?

toFloat() and float() are exactly the same, and this is true for all the other band type cast operators like int() and toInt(). You can use either ones, but if you're looking for a recommendation, I'd say to always use the ones with to, to be a little more specific about what they're doing.

(It happens that the versions without to prefixes came first and the prefixed ones were added later, but the precise rationale for adding the second set of names has been forgotten. Of course, it'd be simpler to have only one set now, but since both are in use, removing either would break some scripts without much benefit.)