I am not a 100% sure, but I believe the MX136 was that last Pre-pro to have a true analog in analog out circuit path. My understanding is the MX150 runs everything thru the digital processing to the outputs. I guess the thought is to have all signals available to process for room correction. You might give McIntosh or the guys at Audio Classics a call to confirm.
i had owned a number of Pre-pros from C39 thru MX135, then changed over to a D2v Anthem Statement. I miss the analog pass thru for the 6 Channel.
As far as making comparisons, I have heard and watched both 136 & 150 but not in a manner to make comparisons. One thing to be concerned with as the MX136 is pretty old in "digital years" so the HDMI , MAY NOT have what you need if intermixing today's video products, but I don't know on that.