Interesting question. On one hand, I don't think Palpatine would skip a chance of nurturing Anakin's darkness and making the Jedi look bad to him. No doubt he would have later said to Anakin something along the lines of how he really wanted to prevent this tragedy but wasn't able to because he was put under immense pressure. Also, it might look suspicious if he refused to sentence Ahsoka despite her being "proven" guilty. Though I guess he could find ways to slow down the trial.
On the other hand, as you say, Ahsoka being actually executed could have evoked Anakin's Sith Lord a lot earlier which might have led to him being excluded by the Jedi Order and to Palpatine's plans regarding Anakin being endangered. I suppose Palpatine could have still tried to recrute Anakin and somehow get rid of Dooku but it would make things more complicated for him.
All in all, I suppose Palpatine would be more careful in this regard and might try to spare her in order to, as you say, encourage Anakin's growing darkness in a more controlled way. But it's a good question