Clevo P870TM Laptop Memory 2x32Gb Single Channel or 4x16Gb Dual Channel

[GoodQuestion]

Hello TechInferno Fans!
I'm a newbie to the site, but not a newbie to computers.  I've been working on them and with them since 1996.  I have always been a bit confused with RAM memory specifically the following aspect.  I'm building a Clevo P870TM (a.k.a Sager NP9877, Eurocom SkyX9C) laptop with an i9-9900k Intel CPU.

The CPU Memory specs are as follow:
Max Memory Size (dependent on memory type)    128 GB
Memory Types                DDR4-2666
Max # of Memory Channels            2
Max Memory Bandwidth            41.6 GB/s
ECC Memory Supported            No

Laptop Service manual states the following:
Four 260 Pin SO-DIMM Sockets Supporting DDR4 2666MHz Memory (The real memory operating frequency depends on the FSB of the processor.)
Memory Expandable from 8GB (minimum) up to 128GB (maximum) with i9 CPUs.  Compatible with 4GB, 8GB, 16GB & 32GB Modules. Supports XMP 2666/3000 MHz. XMP is processor dependent and works with four DIMMs in dual channel or two DIMMs in single channel.

The computer has four memory sockets for 260 pin Small Outline Dual In-line (SO-DIMM) DDR 4 type memory modules.  The total memory size is automatically detected by the POST routine once you turn on your computer.  Note that four SO-DIMMs are only supported by Quad-Core (and greater) CPUs; Dual-Core CPUs support two SO-DIMMs maximum.  Two primary memory sockets are located under component bay cover (the bottom case cover), and two secondary memory sockets are located under the keyboard (not user upgradable). If you are installing only two RAM modules then they should be installed in the primary memory sockets under the component bay cover. Note that the RAM located under the keyboard is not user upgradable.

I have no idea why the manual states "not user upgradable."  The only thing I can think of is if the memory modules under the keyboard were soldered in place.  Then they are not user upgradable.  The CPU I have is an eight (8) core CPU, which means it supports four (4) SO-DIMMS in Dual Channel or two (2) SO-DIMMS in Single Channel.  

16GB mem specs:  Capacity: 16GB   Memory Type: DDR4   ECC: Non-ECC   DIMM Type: SODIMM   Rank: 2Rx8   Speed: 2666   Voltage: 1.2V   Density: 8G   Depth: 2G   Pins: 260 pin
Width Height: x64/LP   Width: x64   Die Generation: 4th   Package Type: Flipchip   Die Package: SDP   Organization: x8   CL: 19   TRCD: 19   TRP: 19

32GB mem specs:  Capacity: 32GB   Memory Type: DDR4   ECC: Non-ECC   DIMM Type: SODIMM   Rank: 2Rx8   Speed: 2666   Voltage: 1.2V   Density: 16G  Depth: 4G  Pins: 260 pin
Width Height: x64/LP   Width: x64   Die Generation: 2nd   Package Type: Flipchip   Die Package: SDP   Organization: x8   CL: 19   TRCD: 19   TRP: 19

The obvious differences between the two type of memory are the specs on density, depth and die generation.

I would like to buy 2x32GB memory sticks for $168 as opposed to 4x16GB for $192 and spend the difference on sea salt caramel ice cream (seriously).

What are the PROS and CONS of 64GB in the Primary Single Channel (2 memory sticks) as opposed to 64GB in Dual Channel configuration (4 memory sticks).  I really appreciate your help. PS (future questions should be WAY shorter than this.)

[GoodQuestion]

After 48 hours of reading almost every thing I could find on RAM I've answered my question. TWO sticks (minimum) of RAM exactly same specs are needed for Dual Channel and they have to go into the Primary Channel. This will be cheaper than 4 sticks of RAM. It will also be faster as the Memory Controller that feeds the data to the CPU will have only 2 sticks to pull data from as opposed to 4 sticks plus there is the added benefit of using less electricity (which is minimal) and less heat is generated and fewer points of failure. PLUS the EXTRA bonus, if you want to add more memory, you're not throwing/selling anything just buying another stick of 32GB (keep specs identical). Speed difference is negligible. So I purchased 2 sticks of 32GB RAM (for a total of 64GB) for $178 including tax and shipping.