So... Can't decide what's best! Flat mana regen, scaling mana regen, or armor (or just some plain scaling health! :O). I've noticed on a lot of high elo streamers that they're using armor seals on their mid AP builds. I guess Ahri doesn't need it if she gets some early dorans and blue, but why the huge popularity of armor? Especially since it's not that much and most ADC will have enough arpen to just ignore those seals.
I always used hp per level. Armor is good if you can dodge the skill shots of the enemy leaving them with to auto-attack.
I usually play Ahri with flat AP quints, Mpene in red, ap/lvl in yellow and flat MR in blue. It's a nice mix of power and MR to go against any AP mid. If i happen to get matched against an AD in mid, I use a runepage that's all Armor and HP. When you are matched against an AP in mid, you usually start with dorans anyway and got some good MR to start with, so you don't have to worry about armor until the enemy ADC poses a threat (Ahri has 74 base armor by lvl 18)
it's proly even better to go with MR/lvl in blue, but I already had the flats and didn't bother with buying the per lvl ones as well. Also I wonder why Tvar liked my post
MP5, since blue buff now scales off your actual mana regen as opposed to a flat amount it really does help you. Ahri is nothing without her harass and combo, and she does have mana issues earlygame. I run a pretty standard page on her (Flat AP quints, mpen red, mp5 yellows and mr blues)