This article from the BA Times helps answer some important questions about the general elections in October. And by the way, it may only be the 27th of December 2018, but those elections soon creep up.
The low-down is that Macri would fare better in a fight against Cristina Kirchner than with Massa, but Vidal would probably do much better in either scenario. This pretty much answers the question over whether it would be 'convenient' if the ex-president was imprisoned before the elections.
It seems that Massa may be the anointed one, so to speak, although he seems well equipped to hang his hat wherever it's convenient to him. he's very bland and seems to trot out the same old boring rhetoric every time he's on TV. Dull and uninspiring doesn't quite describe him.
Macri isn't exactly a great performer either and many of his promises have fallen flat - poverty has in fact increased during his tenure - you know the rest.
Vidal, on the other hand, has far more passion and delivers what she promises. She'd get my vote over the other three.