Definitely Shamrockn'. I have not done Running of the Elk myself, but my wife. son and dughter-in-law have all done it. It is a flat course, but also a very boring course. Shamrockn' starts outside Raley Field and goes through downtown by the Capitol, on the American River Bike Trail, through Old Sac and ends inside the stadium. You cross the tower bridge twice so there is more to see anf much more variety on the Shamrockn course.
The only thing I like better about Running of the Elk is the medals are better, but the Shamrockn shirts are better so its a toss up there.
My wife and I decided all our races next year will be ones the we have never done before so I won't be doing either of these. So far we are planning to do the Tinkerbell races (all three), a
12 hour endurance run,
Running with the Bears by Lassen (haven't decided between the half and full yet), the
Portland Marathon, possibly
CIM and a marathon sponsored by the running group I coach in if it happens next year. If I do the three marathons as planned I will qualify for Marathon Maniacs.