I'm not talking about presenting them as characters with their own movies - that doesn't appear to be on the cards for either of them, at least not before Avengers 2. I'm just talking about each of them as characters who could be inserted into the franchise to add to the Avengers lineup.
An African ninja whose origins are left largely mysterious (or heavily simplified) is an easier sell than a female hero who would likely appear massively overpowered compared to the rest of the cast. Right now the only meta-human heros in the MCU are Hulk and Cap. Ms Marvel can boast hammer and tech free flight, energy absorption and projection; essentially Superman-lite powers that combine the skill sets which already serve to differentiate between Thor, Iron Man and Cap.
Even if they replace the Kree with the Chitauri, make Carol Danvers a SHIELD agent, and call her Captain Marvel, she's still a lot harder to shoehorn into the franchise than a guy who can pop up out of nowhere with an incredible brain and physical skillset that doesn't need to be explained beyond a commitment to training. T'challa can be thrown in anywhere, at any time, and he's instantly magnificent. Danvers has to be established as a regular soldier, then given her powers, and probably an adjustment period as she learns to use them. Since Marvel doesn't seem all that keen to toss her into the mix at this point, and since they need to diversify beyond Falcon, I see T'challa as a much more likely prospect.