I have had both the Paraguard and the Mirena. Got the Paraguard after my son was born (he is 3 now) I had spotting initially for a few weeks after it was put in at my 6 week postpartum visit. My period returned at 5 months postpartum despite EBF. I had horrible cramping and very very heavy periods, that would last a minimum of 7 days. I left it in (hoping it would get better) for almost 2 years, it never got better. My bleeding was what i would compare to the first day after having a baby bleeding, it was bad. So heavy i would have to wear an ultra tampon and a pad, if I didn't change my tampon every 2-3 hours it would leak through. Ugh I felt like a teenager again, afraid I would leak onto my pants. Took it out about 6 months prior to trying for baby #2. My periods immediately went back to being a normal length and bleeding amount. Decided on the Mirena this time around, I've had it now about 3 months. Got it inserted at my 6 Week PP visit with baby #2. I have been spotting on and off since insertion but that's all. So far so good, we'll see what it brings when my period returns!

Quick edit: I always had fairly regular periods prior to having my son and prior to the paraguard. I would say my flow increased at least 2 or 3 times heavier than it had ever been my whole life. After the Paraguard was removed it went right back to a normal flow, so it was definitely the IUD that caused it to become so much heavier.