RimBlock, WOW some very broad brushed comments, which are not factually correct as generalizations you wrote. I am not interested in which one is better or worse, but just to clarify some misconceptions..
There is now a very good knowledge on the C6100 in the forums, possibly due to their availability and cheap price a year or so ago, ( the early learning curve was steep from my observations, but now hte product knowledge is very mature) but much less so on the SM multinode options. ( I am no expert ) These boards some in a myriad of configurations, and the chassis in 2 or 4 node. The boards come in 2011 (v1 and V2) and 1366 formats as well as AMD!!, you can buy new 2011 boards as a slot in upgrade and a new LGA2011, PCIe 3.0 version is under 380 at a rather higher priced ebay seller.
The boards / Chassis come in 2 basic types , those with an ATX style power connection, one on either side at the front of the board ( similar to the Intel and ASUS versions, which all appear similar?) There is another board / chassis version which does not use the ATX style connecter, but the Power Supply has what you could say is a quick connect ( aligned , so you can remove boards and PS eaily and not require any tools etc. ), and the backplane acts as a distrubution board as well is drive interface, these backplanes are available for under 200 on ebay, ( or more if you wish to pay more)
There are also versions which have a good number of slots.. as this one in table below, starting with the option of 3 x PCIE 3.0 * bit riser card option plus several other slots for addon boards, as well as builtin IB etc.
Also most of the option / configurations are also available in 1366 5600 series boards or 5500 if one wishes, I picked up 5600 series 1366 board and tray with IB / QDR built in to board, plus a 16 bit slot which can either be a single card or mone came with riser with 2 slots available for 8 bit PCI2 2.0 cards.
Much of the options we have as second hand users is what is available at the time we need something, and the life cycle of the products sold 2 and 3 yrs ago!! The reason I went for the SM multinode was 2 fold, (am no rocket scientist, but have spent a life sending very high tech electronic packages into oil and gas wells upto 400F) and wanted 2 nodes which used full 2 U height, and I found the workings, bios, systems much less of a mystery than some of the larger brands like HP and DELL. An example is changing fans to a lower speed version is very simple with options in Bios and 4 pin fans connectors as used in all standrard type of current motherboards) So simplicity was a major decision process. in my case..
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[COLOR=#003366 !important]1. Dual socket R (LGA 2011) supports
Intel® Xeon® processor E5-2600
and E5-2600 v2 family[SUP]†[/SUP][/COLOR]
[COLOR=#003366 !important]2. Intel® C602 chipset; QPI up to 8.0GT/s[/COLOR]
[COLOR=#003366 !important]3. Up to 512GB ECC DDR3, up to
1866MHz; 8x DIMM sockets[/COLOR]
[COLOR=#003366 !important]4. Expansion slots: 1 x24 left hand riser
+ 1 x8 for SMCI storage AOC
(support up to 3 add-on cards)[/COLOR]
[COLOR=#003366 !important]5. Single port IB (QDR, 40 Gbps)
w/ QSFP connector[/COLOR]
[COLOR=#003366 !important]6. Intel® i350 Dual port GbE LAN[/COLOR]
[COLOR=#003366 !important]7. 5x SATA2 and 2x SATA3 ports[/COLOR]
[COLOR=#003366 !important]8. Integrated IPMI 2.0 and KVM with
Dedicated LAN[/COLOR]
[COLOR=#003366 !important]9. 3x USB 2.0 ports (2 rear, 1 Type-A)
[/COLOR]