Stuck on which one to get on Ahri. Help?

Neither?

The one with the useful active

Neither.

Open boots + pots,

Ahri's core is WOTA and Rylai's. After that, you nearly always want a Rabbadon's for the AP. By then, you probably need some MR or defense, depending on who's fed and their team composition. Usually Banshee's Veil is a good choice. With that, rylai's and hextech you've got good defensive stats and sustain, plus your ult is an awesome escape.

At that point you've got...

Sorc Shoes
2X Doran's
Rylai's
Rabbadon's
Defensive item

At some point, if they have a lot of MR, you want to have gotten a void staff. If not, now's about the time when you'd do that because they WILL start stacking MR.

That leaves you with one more slot when you sell your other doran's. The game's probably over, Abyssal's probably going to fill that last slot, especially if you went GA as your defensive item.

If you must use either, get DFG. That little bit of CDR and AP on Morello's pales compares to DFG's active.

But since Ahri isn't really a single target burst specialist, she doesn't need it in the first place.

For the sake of not wanting to create another thread, which one for Veigar? While I know Deathfire has extra burst, Ive been testing our Morello's for the CDR and slightly more AP.

DFG on Veigar all the way. It's core on him.

Both are getting changed in the patch probably. So dont hold your breath