The answer seems to be leaning toward the shorter theory. During the recent symposium "The Valley of the Kings Since Howard Carter", Dr. Geoffrey Martin of Cambridge University discussed his team's clearance of KV57, the royal tomb of Horemheb [his commoner one is at Saqqara]. The tomb in the Valley of the Kings was found by Theodore Davis in 1908 but the well chamber and some back rooms had evidently never been fully cleared. Martin's team has now seen some ostraca that indicate the length of Horemheb's reign was around 14 years--and not the 27 that some wished to attribute to him.
However, we will have to wait for publication of those ostraca to find out what they say. I, personally, have written in the past that, given the unfinished status of KV57, the duration of 27 years was not very likely. Even 13 or 14 years seem long enough to have finished that tomb. Even the excavators' debris had not been removed and was still present in KV57 when Davis and crew discovered it. A wall in the burial chamber had not been painted but left in its state of excellent line drawings. [What was the great hurry so that this wall could not be finished?] Horemheb has been accused, now and then, of having appropriated the reigns of some other kings, mainly in order to account for the longer-reign-theory. Now it would appear that the only possibility is that of Ay, whose own duration was short. We know that in Year 8 of Horemheb Maya the Treasurer was inspecting the robbed tomb of Thutmose IV--and I believe that is the earliest date of any activity in the reign of Horemheb, who usurped some of the monuments of a predecessor, Tutankhamun.