Spoken like someone who hasn't tried one or perhaps both of those projects

Kiro is collaborative, but SpecKit is a bunch of templates and then wishes you good luck in your journey. It honestly reminds me of those unified process templates, which I guess all of those are great if one needs some structure to organize ones thoughts

As an alternative, SpecKit is also 0.x release so maybe in 9 months it'll be useful - or overcome by whatever 'ooh, shiny!' follows it