IUI or IVF

I need advice. I had a miscarriage at 15 weeks a month ago that broke my heart. But we are willing to try again. We did three iui and were successful on the third, which was a medicated cycle. At this point would you recommend doing another iui or move on to IVF. I'm 36 and my partner is 38. This will be our first child. I feel conflicted because we were successful with IUI but I'm worried about all the time that is going by. We have been TTC for three years.