I'd have to say the defender should win ties. It seems now that mages with mainly astral should be scripted to magic duel always. Odds are if he has equal or worse astral mages you kill all his mages and laugh, and if he has better mages he probably wants to magic duel you himself and beating him to it increases your odds.
Astral duel should be a fairly risky venture under almost any circumstances (unless you have an astral-10 mage or something
) Keep in mind this is a direct attack on enemy mages. Although I honestly think it might be better if the attacker wins on ties BUT the defender gets to use FULL magic skill or +1/2 other magics or something. It makes sense to me, and it would make some of the astral 1/mage/priest units a good deal more useful against astral nations with cheap mages that get a greater than 50% chance to kill them. But that's just my opinion and I freely admit I'm not too experienced.