The primary difference between dry-transfers and decals is that dry-transfers (and it’s hard to maintain a straight face while I write this next bit) transfer dry to a surface and decals transfer wet. [EDIT: Unfortunately there is now another difference between decals and dry-transfers. Decals are still being made, dry-transfers are not. The last company that manufactured the chemicals necessary to produce dry-transfers has stopped producing those chemicals. Evidently computer graphics have replaced the role of dry-transfers. Bastids…]
So why bother with one instead of the other? Detail.
My experience has indicated that dry-transfers offer more minute details than decals do. (I assume that’s predicated by the limitations of the manufacturing process.) When I want small details I’ll go with dry-transfers. When small details don’t matter, I’ll use decals. Sometimes there are graphics that are only available in decals, sometimes the graphics are only available in dry-transfers, so that can also drive the bus.