Happened to use that combo recently as I had Ignitors then decided to try out the Ikons.
Only got one Ikon to start off with and put it out back while I waited for the 2nd tyre.
The Ignitor is better for loose conditions, especially if it gets wet. It's a super dependable tyre, if a little slow in comparison to the Ikon.
Agree about the volume comment - the Ikon is a bit bigger and will be a smoother tyre on rocky terrain, but if it is loose-rocky, I would be keen to get the Ignitor up front for sure.