Simple question: how large have you ever printed from the M9 (or the 1Dx )?
The underlying question is, how much resolution do you want at final output size? Excellent mural size output can be produced from your files, but is the quality good enough? It needs an objective determination of what quality you want.
This can be determined by doing a bit of simple math, and an assumption. Let's assume you want to reach what's considered to be quite a good output resolution of 5
lp/mm at reading distance. Then that's the goal. If you can live with a lower quality you'll be able to enlarge more and lose some resolution, otherwise you'd have to prevent people from getting too close.
We now need to determine the amount of detail that a given sensor can produce (theoretical opimum).
1. The M9 sensor has an absolutely maximum resolution (AKA Nyquist frequency) of approx. 5212px / 36mm = 144.78 lines/mm or 72.39 line pairs/mm. That means that you can magnify the 36x24mm image 72.39/5
= 14.48x, which is 521 x 347 mm (divide by 25.4mm to get inches).
2. The 1Dx sensor has an absolutely maximum resolution (AKA Nyquist frequency) of approx. 5184px / 36mm = 144.0 lines/mm or 72.0 line pairs/mm. That means that you can magnify the 36x24mm image 72/5
= 14.4x, which is 518 x 346 mm (divide by 25.4mm to get inches).
So, technically there is no significant difference, therefore it will boil down to what the specific lens qualities can bring you for the specific focal length and aperture combinations that you will use, and what shooting convenience is best for the shooting conditions you will face.
There are 4 slots for 30 x 40 prints (which no one went for, hence.... enter the dragon..).
Do you reckon the 18MP native file would be enough for that ?
Is that 30x40 inches? Then the step from 13.6 inches on the short side to 30 inches, leaves you with 45.4% of the 5 lp/mm goal. So there would be a drop in quality, unless you consider stitching or a 2x larger sensor size (physical size). You'll need to use a very good upsampling program to optimize the quality you could get from your current gear anyway.