Those two scopes you mention are pretty distant in price points. A BDC rectical is a bad choice for known distance target shooting at a range, period. Scratch the M308 off your list.
You should either dial for the known range, or hold over with mil or moa stadia.
BTW, I think your usage model needs a bit of review. an LR308 in an SASS configuration has a lot of capabilities, but it sounds like you are not planning on optimizing them based upon your scope selection. If you truly have no intention of using that as a personal defensive rifle, in which range estimation via reticle could be a lifesaver, fine. If you truly have no intention of shooting past 600 meters, even though the rifle can go to 900m, fine. So, you spent an extra $1000 on a 308 range rifle, to get a semi auto, when you could have just bought a bolt gun with better accuracy, fine. If you are fine with all those assumptions, then don't bother to spend money on first focal plane or a Mil/Mil solution.
If it was me, and I had $750-$900 to spend on a Mark 4, I wouldn't buy one. I'd buy a Bushnell 3-12 FFP G2, or a Vortex PST FFP Mil, depending upon how much I felt I needed the illumination, the option to go 24x and a "kinda" zero stop of the Vortex. You'll note that the veteran Calguns shooters (vs. collectors) above have also suggested these solutions... with good reason: experience!
But for your stated use model of 300-600 meters, known distance range use, I'd suggest saving a whole bunch of money and going with a Bushnell 5-15x40mm, a Nikon Buckmaster 6-18x40mm, or maybe even a fixed power Bushnell 10x Mil/Mil (depending upon what you define as a "target" (Type E targets vs. precision bulls-eye). My rule of thumb on magnification is a min. of 1x per 100m/y for Type E, 3x per 100m/y for precision; YMMV.