I think one of the scariest things about having a geass that can see a future event is if the event being witnessed becomes some life altering thing. What if you could foresee the end of the world? Or the death of a loved one? How do you go about stopping it and can you ever really? I think such a powerful thing has the ability to drive anybody insane. I now wonder if Bismarck was able to tell that he would eventually be defeated, and Lelouch would at some point sacrifice himself? For a side character, he had the potential to hold alot of knowledge that the viewer is never told about, especially since we barely know the full explanation of his geass and its entire limitations. I mean, what if it can see more than a few seconRAB like how Rolo was able to continuously use/ extend the time his geass worked for by putting more stress on his heart?
As for which Geass I'd personally pick.... I'm stuck between either Rolo's or Lelouch's. Rolo has the disadvantage of a harmful side effect, but then as we've seen with Lelouch his could lead to a similar situation. I'd probably end up choosing Lelouch's geass, because there are still ways to manage it with out going overboard.