I've had the Mirena IUD since December 2013, and I'm super happy with it. No periods after the first month, and it doesn't have any of the side effects that I had when I was on the pill (weight gain, bad skin, insane mood swings). Insertion was painful, but I'm sure you have already done your research on that front. I was able to go to class later that day, and I was running pain-free a week later. I did have cramping that ranged from mild to pretty severe for the first month or so, and sometimes I still spot after a particularly hard workout, but it's easily worth it for five years of worry-free B.C.
I would be a little nervous of the Paragard as a runner. I have heard way too many accounts of insanely long, heavy periods, and I'd be concerned about becoming anemic or just feeling too crappy to run 25% of the month.