On the other side of the argument, some situations arise in which it's better to cut your loses. If you have reason to suspect you may be fired, your best strategy may be an early resignation. Then you explain to potential employers that the job did not develop as you had expected. I realized that the picture for me would be quite different. B. A mismatch.
Nothing is really wrong but the other offer is stupendous.
If you scoot out, the prince of a recruiter is left holding the bag. Most people will take the unrefusable offer and run. But I think you owe the recruiter advance notice and an offer to help find and orient a new candidate. Sorry, the volume of mail makes personal replies impossible.