Personally, I automatically think Mee-la when I see the Mila spelling as Mila Kunis is so well known, but I would always check first as I have known many to be My-la instead. Understandable really, due to the pronunciation of Milo/Miles, and the fact that Lila is usually Ly-la rather than Lee-la.
Personally I prefer My-la, as it's softer. Mee-la just makes me think of mealworms! If looking for My-la though, I think the Myla spelling avoids confusion. I've never heard it said as Miller, personally. Maybe that's a regional thing?
As you have probably realised by now, it's a problematic name which she's destined to have to spell out and have pronounced incorrectly all her life (similar to Maya, Esme, Sara etc). Some names are definitely more complicated than others!