Both options would be regrettable, however since 'neither' isn't an option, looking at both companies track records, I'd have to go with Sony.
Microsoft's one advantage is also releasing their titles on PC these days, but there's no guarantee that will continue indefinitely, nor that their current business model, focusing ever more myopicly on service and catering to their existing audience will pay off.
Conversly, their track record with managing studios, maintaing IP's and cultivating new games has been so abysmal that they were forced to buy a whole new set of studios to try and fix their failures.
Now, those acquisitions may well prove to be highly fruitful in the short term, but the sad reality is that the odds are that the same mistakes and lack of focus on creative vision in the games they Greenhithe will continue, leading to the WB studios and series either being shuttered or reduced to churning out the same tired and underwhelming games again and again and again.
There's also the issue of whether a PC release means Steam or just the Windows Store, as the later combined with the Xbox audience would not equal the number of people that would be buying the game on Playstation.
Now Sony on the other hand, they have a proven track record of both consistently giving their studios the freedom to make the highest quality titles they can, and encouraging those studios to take risks in creating new IP.
What's more, Playstation is a global brand and the best selling platform for most of WB's titles. With the sudden recent change to porting to PC also, the audience a Sony acquisition could well make the Xbox audience completely irrelevant, although this is even less a certain trend than Xbox and PC.
Even without PC in the mix however, and ignoring sales, I'd still say Microsoft would more likely be a slow death for the WB studios, while Sony could potentially mean rising to new heights.