I wanted the copper IUD until my GYN, who I trust very much, told me that the likelihood of becoming pregnant and miscarrying multiple times is far greater on the copper than the Mirena. She said that the minute amounts of hormones shouldn't change my cycle or be harmful to breastfeeding. I have only come across 1 person who had BFing issues.
With paragaurd, your period is also likely to become heavier and more painful and with Mirena, it might become lighter than normal.
That's how I chose which one to use. Now, I just need to find out how long I can expect this spotting to last. I got it on June 29 on CD4 and am still spotting.