Lets compare AD and AP itemisation

Penetration(kappa) Mages definitely have better % penetration,note i said % penetration because they have the total penetration HOWEVER THEIR FLAT PEN IS GARBAGE excluding runes 2 magic pen items total into 30

AD there % pen isn't that great due to it being bonus penetration on last whisper,mortal reminder and that other one I can't remember,BUT THEIR FLAT PEN IS GREAT with totalling a whopping 60 before runes

here is where the problem on the penetration side,a lot of the time assassins will build all 3 of there flat pen items,but mages won't ALWAYS buy their assorted flat magic pen,because some people could choose lucidity and not all mages can benefit well from liandries

Defense are the mages options,in terms of defensive utility id say mages overall BEAT AD defense on the utility side especially with the 25% attack speed removal coming for

AD defense options are a lot better on the survivability side ,lets look at the facts they have shield,lifesteal,armour pen,cleanse and even spell vamp..

Grrrr this is tough to choose generally zhonyas is a very very strong item but here is the issue AD penetration would just beat through the armour from zhonyas, not to mention mages have a tough time bursting through 2 layers of shields,also the lifesteal can be quite frustrating..

Prices.. you already know this AP items have outrageous costs(the zhonyas price is acceptable it is very strong)

AD items have to much value in 1 item for their price

AP itemisation is quite...lackluster i guess and AD itemisation is better