Going by feats Wanda seems to have greater raw power (we've seen her overpower Thanos, destroy the mind stone, casually throw around huge alien machines etc) and her shields would block a direct attack.
On the other hand, Strange has a huge array of spells he can use to outmanoeuvre her (e.g. portals, flight, shields, body duplication etc) and take her out (e.g. energy blasts, conjured weapons, binding spells, trapping her with a portal).
Since they're both easily powerful enough to take the other out as soon as they land one clean hit, I think the deciding factor is skill and with that in mind, I vote Strange.
To be fair, I do think IW Iron Man could probably beat Loki if you take the extremely OP weapons Loki had in Thor 1/Avengers 1 out of the equation.