We picked guardians in our family based on their own home set up, and their proximitiy to us, their capability to look after another child and the fact that they let us know that they would be happy to take on that responsibility.
They are financially able to take on another child, and ensure he's educated right to third level, there's room for him in their home, he would be around his own friends at school, wouldn't have to move, and they would ensure that our house a few doors down was maintained until DS was old enough to live in it himself. They'd probably rent it out and bank it for him, and not take a penny of it, knowing them.
He already went there after school a few days a week and is treated like the youngest of the family, one of their own, so it's not a new environment for him.
If the worst happened to us, they would ensure he thrived to the best of their ability, had every opportunity that their own kids have.
My sister would do the same, but the reason she wasn't picked was just purely geographical. If DS was orphaned overnight, to uproot him and move him to where she lives would be even more trauma and upheaval than if he went up the road to the cousins that he's like a baby brother to.
Honestly, I'd pause before I nominated someone who was in a recent abusive relationship to rear DS - and I say that as someone who was in an abusive relationship myself. Just because I look back having had years of working on figuring out why I was susceptible to that kind of relationship in the first place and see how much dysfunction there actually was in my life that I just didn't see as abnormal at the time. If it was years down the line and I could see you going on to have stability and a healthy relationship, sure. But not six months into your freedom.