Mazal tov on the birth of your baby!
Both the copper IUD and Mirena are associated with spotting following insertion. With a breastfeeding woman, the spotting may be somewhat less, but there is no way to know for certain how any specific woman's body will react to either type of IUD.
Spotting from the copper IUD tends to resolve more quickly following insertion, although when a woman menstruates her bleeding is often longer than without the IUD. Spotting from Mirena can continue for up to six months after insertion. However, after an initial adjustment period, women on Mirena may not have a blood flow for years. This is a judgment call for each couple to make.
We suggest waiting until after mikveh immersion postpartum to place either IUD, so that you will not need to count shivah neki'im while spotting post-insertion. Spermicide is often sufficient contraceptive protection for a woman who is fully breastfeeding 4 weeks postpartum, and and may provide natural contraception between immersion and insertion, though this should be discussed with your physician.
It is important to review the laws of stains, as by reading our site's articles “Stains” and “Toilet Paper“, to avoid becoming niddah unnecessarily, whichever course you take.
Please write back with any further questions.