First, I think we need to ask two questions. The first is in literature or among the folk? And the second is where in England/Britain?

When it comes to which inspired more in writing (and by implication if we can allow that implication, who had a larger readership?), I think we'd have to give the nod to Arthur and his court. Far more was written on the Arthurian realm in England before (and after!) 1500.

Then we need to consider what were the folk more apt to tell stories about. That's harder to evaluate because there wasn't systematic collecting of oral traditions going on, but we can get a hint of the possibilities. Arthurian traditions among the folk were most at home in the west of Britain, getting strongest as one approaches Cornwall and Wales where they were most at home in the earliest period. In those areas, we can probably assert that not only was Arthur more popular, but the farther west one went, the more likely that people would not have heard of Robin Hood.

An excellent treatment of the Robin Hood material appeared in 1981 by J C Holt (in a book not surprisingly title "Robin Hood"). He begins his discussion of the Robin Hood legend by telling us that:

Practically all that is known of the medieval legend of Robin Hood is derived from five surviving poems or ballads and a fragment of a play. The earliest of these ... is found in a manuscript collection written about 1450. ... Another tale [was written] shortly after 1503.

From this we can imagine two things: Robin Hood was later to take hold (Arthurian sources are spattered throughout Medieval literature), and it was not as widespread. Early English literature is full of outlaw heroes, most particularly, Hereward the Wake, Eustace the Monk and Fulk fitz Warin, and from this we can see that the motif enjoyed a great deal of popularity in Britain. We remain entranced by this motif - consider Jesse James and also Oceans 8 through 163. Grettir the Strong is something of the national hero of Iceland - so this may be international! We all love a good outlaw story, and this energy clearly put a lot of wind behind Robin's sails. It is possible that at some points before 1500 and in some places farther away from the west, Robin Hood (or some early precursor to the character we now know) was better known among the folk than King Arthur. But even in the north of England, we have the story of Sir Gawain and the Green Knight. And even though Chaucer gives us an outlaw (not Robin, but some see a connection) with his "Tale of Gamelyn," his uses of Arthurian material are numerous.

On all counts, I think we have to give the nod to Arthur and his court - both in literature and among the folk with the stories they told.