Heir time again!

I'm about 90% sure of who I want as heir for the next generation but I thought I would see what others thought. My candidates in the last heir vote were neck and neck and I ended up playing both. Between them, they have 3 boys and 1 girl. The girl, of course, isn't even in the running and the youngest boy's nose is too close to his mouth so he's also not in the race. The candidates are Duff, son of Euan and Greg, and Niall, son of Fife and Brett.

On the left are Fife and Brett with their teens, Beitris and Niall. Brett and Euan with their boys, Duff and Tam, are on the right.
https://farm1.staticflickr.com/793/41248328701_89aef4fcb7_o.jpg

The boys are a little awkward looking in their teens. Here's a closeup of Duff, expressionless so you can see his features without distortion.
https://farm1.staticflickr.com/817/40322707355_4c9f776704_o.jpg
Here's a closeup of Niall.
https://farm1.staticflickr.com/800/39441887780_20a3032223_o.jpg

I aged them up in CAS to show you what they will grow up like.
https://farm1.staticflickr.com/865/39441885000_80d6f97b09_o.jpg
https://farm1.staticflickr.com/869/40354977955_60c287b452_o.jpg

I won't tell you their traits because they are irrelevant as I only play for genetics.

Who do you think should be heir?