Given a known conflict, and time to plan and arm, I believe the Altmer would come prepared for anything the Dunmer could throw at him, and then some. The kind of arrogance which the Altmer possess only comes from long years of BEING the best, and knowing it. Staged combat in advance: Altmer 3 out of 4.
The Dunmer also have a lot more internal conflicts to deal with, and any organized response is likely to degenerate into a series of small independent actions which can be dealth with piecemeal by any opponent, much the way that Tiber Septim handled them. Their mastery of assassination and deceit, on the other hand, are to be feared. War between the two: Altmer overall, but the Dunmer would pull enough shadowy tricks behind the scenes to make it a costly win for the Altmer.