Diggersby IV dilemma

So I’ve been walking the first Diggersby for XL candies for a while now and made it best buddy along the way. I just caught a new Bunnelby that’s rank 7 in the great league but requires like 50 more XL candies and isn’t best buddy. Is powering up the first one a dumb choice? Is a rank 42 and rank 7 a big difference or has no true difference?