There are two kinds of IUD, the copper kind can last 12 years and has no hormones, which is important for some people.
The plastic kind lasts 5 years, but has hormones.
I have the plastic kind, so I will speak about what I see as it's benefits over an implant.
1. It lasts longer. I am poor and uninsured. I was able to get my IUD inserted for free, and now don't have to worry about it for 5 years. The implant only last 3, and if I had had one inserted instead of an IUD I would need it replaced before I am likely to have regular healthcare.
2. It costs less per year. I didn't pay for mine, but I likely will when I have it replaced. The implant and IUD are similarly priced, but since my IUD lasts longer, it is a better investment.
3. Other than the pain of insertion/removal side effects and risks are similar. I can't imagine implant insertion feels great, but I concede it is probably better than IUD insertion. For an extra 2 years of coverage, it was worth it. Other side effects/risks are mostly hormone related and they are similar between the two. the implant gives a larger initial burst of hormone and has more users that report emotional side effects, but for most of the time hormone levels are similar.