SPF Record with or without plus

Technically, yes, there is a difference. According to the SPF Record Syntax, a plus sign (+) indicates that the record should pass, no questions asked. The absence of a plus sign, or the inclusion of a question mark (?) indicates a "neutral" status - i.e. the domain owner is not saying 1 way or another whether that particular record should pass.

However, the end result is (almost) always the same: The email will pass.

The SPF record syntax summary, right at the top, states that

If a mechanism results in a hit, its qualifier value is used. The default qualifier is "+", i.e. "Pass".

So it seems clear to me that there's no technical difference between mx and +mx.