Obviously, it depends on a number of factors, but if we're taking that one issue of "stronger or weaker" in a vacuum in terms of playing ability, I'd take the weaker ally.
For one, they tend to be more easily persuaded or more trusting. They seem to be more loyal, or at least if they aren't, their stabs aren't well-planned or executed. Finally, they are much easier to stab than a strong player is, and if you intend to solo, you're probably going to have to stab your ally at some point. Along this note, I would assume the weaker player would be less likely to stab you in an attempt to stop your potential solo than a stronger partner would.
Now, I don't want to ally with a complete noob; I've learned my lesson on that. But I would certainly rather an average-skilled ally than one who plays excellent.