Hi,
10500/11900 are using a port-based CLOS, which means that a packet from card 1 port 1 to card 2 port 1 will take 1 path over fabric module x and will always use that same fabric card (unless failure happens of course).
So there is some port to fabric mapping, so assume (just example) that first 4 ports of each card would be mapped to fabric module 1, when you would only connect servers/switches to these 4 ports, then traffic would only pass fabric module1, the other fabrics would not be used (a bit like xbar, but 4 of them connecting to a single line card).
This automatically means that some fabric modules will get more/less load compared to the others.
The 12500/12900 are using virtual output queues, which means that the line card will slice a frame into multiple cells, and these cells are distributed over all fabric modules. So all fabric modules are always used for inter module packet delivery. This will give very predictible switch packet latency, since the load is always shared over all units.
Most people will not ever see this difference in real flows, but this is one of the reasons why 12xxx platform is preferred for core.
10500/11900 CLOS is "easier" to develop, so that will also be reflected in the pricing compared to the 12xxx components.
11900 is essentially a 10500 (IMO) which can only be equipped with the right DC modules (line card buffers and management module on cmw7) while the 10500 can also have more basic line cards, as well as cmw5 or cmw7 management modules.
The mentioned h3c 7600-x is unknown to me, but specs look very similar to 10500, so I guess (based on title 'carrier class') that it will be more SP oriented (so it is possible it will never come to us, but that is just a guess).
Best regards,Peter