Who says it's either or? The whole point of Apple's strategy is to have both. Continuity plays right into that. I won't write a novel or a complex spreadsheet on an iPad, but I can have it on during an entire flight, and not just below 10,000 ft like with a laptop. Or I can make edits to a PowerPoint while in a cab to my client.
When someone advertises something as a productive device, it implies that the device can do everything you need to get actual real world work done all by itself. The iPad lacks that in almost every area. It's a good supplement to production, but that's about it. It's not efficient at all. Another critical aspect to "productivity'.
We have one, and I like it for very light casual activity, but a work machine it is most certainly not. Not even nearly.