Daniel's being overly gracious, but thank you anyway
"I read that main difference between Air Tech and G12 is that G12 has longer spikes which makes is better for steeper slopes. On the other hands with Air Tech it's easier to walk and better in mixed terrain."
Correct. Just pick your poison. I like them both in the appropriate terrain.
Big fan of the steel Air Tech's myself as a general mtneering crampon. And use the G12 on ice up to WI6 and M7. So obviously there is some serious over lap between models.
Big boots? The flex bar thing is a "required" BD invention because their ss plate 'pon's break. Option for everyone else. Grivels don't break as a general rule. I wear 45/46s and have no issues in Grivel or Petzl 'pons. The solid bars will work with stiff boots. But stiff boots are what both the crampons are designed for. Your bigger boots make that a bit of a problem but not much. Keep the solid bars with stiff boots as any crampon will climb better because of it.
Soft boots you'll want flex bars on any crampon you want to keep in one piece.
Crampon fit? Much as I love Grivel. I recently started fitting a bunch of lwt 3 season mtn boots to crampons that are not full clip on bindings which I prefer. These are a basket toe and several different heel atatchments. Of the 3 major brands...Petzl, Grivel and BD, I found only Petzl offering a decent fit on my 5 pairs of different brand boots (Scarpa, North Face, Sportiva, Zamberlan, Salewa). Petzl was the best fit on every boot and even then not a stellar fit by any means. But a good bit better than anyone else for sure! Just an observation I thought worth passing on........