Definitely wait. It always frustrated me growing up when people tried to say it because I would still either need to say it myself to continue or it'd just move the block to the next word, and most times they'd guess completely wrong so it'd just add more words for me to struggle through, with added extra stress cause I felt rushed. But nowadays I don't even try to say it at all anymore, if I block I just reword, and keep quickly rewording until I manage to get words out before my brain gets the chance to think them, that's the only way I can speak.