Maybe i'm a bit biased from playing toplane leona, but for me it's 100% AD.
Sure, she doesn't have direct ad-scaleings, but her AP scaleings are REALLY low aswell, and what most people don't know is that her Q doesn't just reset her AA once, but actually twice(if you rightclick your target imideatly after the shield bash, you will get another AA-reset for a quick triple hit). This means she actually has a high ad/on-hit ratio on that Q if fully used.
Also, for early bot fights, you rarlly have full 100-0 bursts, most fights are a bit more scrappy, and with her high CDs, ad has just sooo much more value in extendet fights
Also, some quick math on her burst if we asume you land E, follow with AA-q-AA and then get another AA of thx to the stun. This would mean her combo has a total ad and on-hit ratio of 4.0
The total AP ratio of her q+w+e+ is 1.1, and unless you use E befor the engage(whiich usually is a stupid idea cause it warns your opponents that you are looking for an engage) you will have 0.4 of that ratio only after 3 sec dealay, which means you could add in another ~2-3 AAs to increase the total ad-ratio to 6-7
If we are very generous to AP, we throw in the ult and say you only get off 6 AAs in the fight, which makes it a 6.0 ad ratio vs 1.9 AP
Now if we look at the support items, it's 24 ad vs 40AP, so the AD-one gives 144 damage to the all-in while the AP one only adds 76, so the ad one is nearly twice as much damage!
Now there are some other advantages for AP as it's magic damage and mr overall is lower than armor, and it's on aoe. However, your goal usually is to lock down and burst a priority target, so the aoe part isn't a big deal, and AD also does allot more after your combo is on CD, especially since the only low CD skill has basically a 2.0 ad ratio thx to the resets.
Also, if you look for an damage option as support, there are 3 items to look at:
, good defense and utillity, but also nice extra damage on the Qs for a reasonable price(rather high end for support, but not totally out of reach)
, lots of damage potential and extra cc for lowish costs, but requires your carry to actually follow up on your engage, so not always usefull in soloQ
, if you have solid magic damage followup from your team, the 15% amp is massive. However, the high cost means it's only a good option if fed or if your team has high magic damge, aswell as your opponents so you also get high value from the MR
Fun fact: the 15% amp gives leonas combo upt to 120damage from base damages alone, without any passive procs(though E and sometimes ult usually hit outside the aoe) which equals 60 APfor her