After the birth of my daughter, I was in the exact same situation that you are in now. I had decided on an IUD but didn't know which one to go with. I had only been on the pill twice in my life before, and both times with a very negative reaction. The hormones in them (I even took a low-dose form the second time around) did not agree with me - I suffered from depression and had a significantly lower libido on both forms. Needless to say, I was very hesitant to do anything else to my body involving an alteration of hormones.
My doctor told me that the hormones in the Mirena IUD were practically insignificant compared to birth control pills. She also thought that because my cycle has always been inconsistent, that what hormones there were would actually be beneficial. I went ahead and chose that one over the copper IUD.
I have been nothing but pleased with my choice, and I have had the Mirena IUD for almost 2 years now. I haven't experienced any of the side effects that I had before with the pill. There was no weight gain, no hormonal changes that I have noticed, and I even stopped having periods altogether (can't complain with that!). It was painful to have put in, and I can't say that I'm looking forward to having it removed some day, but otherwise, I have no reason to complain about it.