Ah, but you see, one of the advantages of having a legislature choose senators is that they wouldn't have to pick between only two candidates, the way the voters have to. They can pick anyone they want.
And as for whether the legislature would pick a better senator, I can't say for sure. For one thing, if legislatures pick senators, it would change the way people would elect their state legislatures. Right now, most people couldn't care less who their state reps are, and in many districts they run unopposed. This leads to a complete subservience of state government to the feds. Giving state governments a tie-in to the federal government would change that.
Secondly, senators (and politicians in general) act the way they do in large part because of the influence of political pressure groups, whom they rely on for campaign funding. This would be rendered unnecessary if state legislatures were to choose them, since no senator would have to mount a multi-million dollar political campaign to get a legislature's attention.