Clinton is a known quantity, yes. But I know that a Hillary Clinton presidency will be 8 years of investigations and recriminations into crimes, real or imagined. I also know that very little will change going from an Obama presidency to a Clinton one.
Trump is an unknown quantity that has said some scary things when taken out of context - but speaks the things people won't say, but think. Is probably equal to our current president in the ability to sell a message. The worst case I see for a Trump presidency is something ineffectual. The middle case, he's a wrecking ball, and breaks up enough to the deadlock, fake culture wars, and stupidity in politics to unite our country against him and on to further greatness. The best case, he's a very effective president and can break down walls, modernize the executive branch and shave off trillions in waste.
In short, if you think things are fine as they are - and only incremental improvements are needed, Clinton is your lady - if you think the system is broken and needs a wrecking ball, Trump is your guy.
I'm confident enough in the fundamentals of our system of government, that I'm not concerned about the future of our country with either outcome.