I have a 200A Square D primary panel mounted 5' off the concrete pad between two studs on the back wall of my garage. I have a 50A CFCI breaker and high quality NEMA 14-50 socket that will be mounted in a 4" deep steel box attached to a stud 4' off the floor ~8' to the right of the panel (center of double garage). My original plan was to use #4 THHN in 1" Reduced Wall Aluminum Flexible Metal Conduit horizontally between the bottom of the panel and the box (I know #4 is not needed, but I have it and the conduit left over from another project). Studs are 2x4 nominal, but measuring them they are 3.25" deep, so the 1 3/8" bore hole though the studs would be 42% of depth violating the 40% max for code, and as I will need to bore through 7 studs I can't double them even if I wanted to).
So I need some help deciding between:
running 6/3 UF (much cheaper than Romex) from the top of the panel though the top plate, 9ft across, then down the stud to the box. For added protection I would encase the down leg to the box in the 1" RWC conduit that I have, firmly attached to the stud. (I believe that NM and UL can be run in conduit where additional mechanical protection is needed).
Running 6/3 MC from the bottom of the panel to the box through the studs. This has a much smaller OD, requiring a 1" bore hole that would be within code on the 3.25" deep studs.
Which is these, or any other approach makes the most sense? What am I missing? I will have the installation inspected so all needs to be to code. Many thanks.