AP Lulu is overall more recommended because she'll turn into a second support late-game - granting a huge shield and hp bonus so AP Lulu mid is a niche pick for those protect-the-carry compositions.
Why just standart support? Even some other Champs f.e. Janna, can be also used in mid as AP Janna.
But back to the question.
I think that depends on your own gaming style.
For my own style, I use Hybrid Lulu rarely but not for the "lol's" only
A straight ADC build (IE PD LW) will almost always have a higher DPS late game than any other build, plus Lulu has low ratios and 2 damage skills.
On-hit Lulu has a lot of sustained damage, but I'm not really sure how it compares to full AP/full AD.
Depend on your item build, runes, and masteries,
But back to the question.
I think that depends on your own gaming style.
For my own style, I use Hybrid Lulu rarely but not for the "lol's" only