Get 12 point. No way could you bring aluminum or 10-point to Denali. Aluminum is not very durable for long-term use, especially if you're walking over rock in them. On Rainier, and I'm sure in New York, there is always rock to cross (e.g. Disappointment Cleaver when the route is melted out). Pretty much any make will be fine...Grivel, Black Diamond, SMC. The attachment type will depend on the flexibility and configuration of your boots. There is no way you can take your flexible backpacking boots on either Rainier or Denali. Probably the only crampons that will attach to a more flexible boot are strap-ons. My old SMC strap-on's are great, but they're sort of hard to find these days. Everyone wants the step-in kind, which should work great on any boot you'll take up Rainier.
It is also doubtful that you'll be using the same boots on Rainier (generally a warm climb) as you will on Denali (frigid and high-altitude), so make sure your crampons can fit over a variety of boots.