Absorbency on both is the same - same layers of terry. The difference is that if you check a bumble you will see it is stitched and then turned - this gives a good snug fit around the legs but the turning does add to any bulkiness of the nappy. Bimbles have the stitching on the outside so are not 'turned', therefore slightly trimmer - also they are only available as popper fastening.
Straight from the lovely lady at easypeasy