Our guardians are our best friends. They also have 2 kids. They are DS's God parents.
The biggest consideration for us was parenting style. I have 2 siblings and while I love them their values are very different to ours. I actually think our kids would flip out living with them. Also, my sister's kids are now adults so she's in a new stage of life. My brother has a teenaged child who he shares custody of with his ex. DHs brother is single and not a kid person at all. Our parents are not an option as they are elderly.
DS actually asked me recently what would happen to him as DD if we died. I said he would go to our friends and I could see the relief on his face. Good choice (heaven forbid it ever happens).
We of course discussed this with our friends before putting it on our will. Our families also know.