The only difference is mostly to look at what they bring to the table:
-Ratfolk would allow you to use touch hex further away from you. At level 6 for example, you would be able to use a hex with a range of touch 30 feet from you. It's practical, if you plan to use a touch hex often. If not, guess you are small...and that's it.
-Tiefling make your witch familiar more resistant to elemental damage, which is okay with pass for human alternate racial trait, you can take the human FCB too. Tiefling have many fun alternate racial traits or variant tieflings, so worth looking into them.
-Human increase their spell repertoire with their FCB and of course humans are ridiculous versatile with the alternate racial traits.
So depends really.
Human for More spells known, an extra feat, and +1 skill point per level.
As for Point buy, I would probably do:
Str: 9, Dex: 14, Con: 14, Int: 19, Wis: 10, Cha: 7
For your First Level Feat, I would take Improved Initiative as one of them.
I do not much like Pass for human as it takes away the Tail, Wings and Scaled Skin options. It also makes you vulnerable to the Charm/Hold/Dominate Person.
All good race choices.
Of the three tiefling is my favorite, though I agree with Louise, skip pass for human. Prehensile Tail, Vestigial Wings, and for me the bite attack are way too good to pass up. THe tail helps you draw and hold items, the wings help with the flight you will want later, and for me the bite attack and a longspear help keep you provoke attacks of opportunity, and if necessary setup flanking for your allies.
Tiefling Point buy: Str: 11, Dex: 14, Con: 14, Int: 19, Wis: 10, Cha: 8
Ratfolk: Ratfolk have a special place in my heart as one of my favorite races. If no one else in your party is one, get the cornered fury alternate racial trait. Scent is pretty useful too. While the added range to a hex is very good, to me the real advantage to a ratfolk is that you can ride a medium size mount, so you never have to worry about space. Let me explain why that is good. If you have a combat trained mount, for you a pony, or another animal from the animal archive, and a couple of points in ride, you can control him as a free action. While you can only attack once if your mount moves, and if you go too fast your casting will require a concentration check, your hexes do not. So your mount can move to within 30 ft of a foe, you can evil eye, misfortune, or slumber them, cackle as a move action, then have your mount use his other move action to get you out of possible danger.
Now, at higher levels your mount will die easily, and other races can do this too, they just need large size mounts, which actually do fit in most places. I just like the idea of a ratfolk riding a dire rat, or a gecko into battle.
Ratfolk Point buy: Str: 10, Dex: 14, Con: 14, Int: 19, Wis: 10, Cha: 8 The strength is for carrying capacity.
Human: good favored class bonus, bonus feat, bonus skills. Hard to argue with that. If you take a familiar that gives you +4 initiative, and spend your bonus feat on improved initiative, you are looking at a +10 or +12 at level 1. Going first can be the difference between life and death.
EDIT: One piece of kind of gamey advice no matter what race you pick, carry around a scythe if you are taking the slumber hex. You don't have to be proficient in a weapon, to get that sweet sweet x4 damage on the coup de grace.
-Ratfolk would allow you to use touch hex further away from you. At level 6 for example, you would be able to use a hex with a range of touch 30 feet from you. It's practical, if you plan to use a touch hex often.
From the PRD: "Witch: Add +5 feet to the range of one hex with a range other than 'touch.'"
If you don't really need that extra feat, I'd go Ratfolk. Tinker is amazing together with the trait Pragmatic Activator and, eventually, the Cauldron hex.
Also, since it's small sized, not only it has higher AC and attack bonus, but can use its familiar as a mount if Mauler, which in turn could intimidate opponents to add a nice -2 to saves before you cast your hexes or spells.