Internal vs. External Sales Training - which is more affective?

I'm currently working with leadership on ways to better develop and train our younger sales reps and our VP wants to hire an outside consultant to get a different voice in the building.

I'm of the opinion that we can teach our reps internally through a mentorship program, but this is also asking a lot of our more experienced reps even though they know way more about our sales process and accounts that we're working with compared to someone on the outside.

Have you had an outside sales trainer come to your sales org and did it have REAL, lasting success?