Two systems and SSD lifetime increase
Memory wear corresponds more to drive usage than to drive size. SSD controller employs wear leveling techniques to spread writes evenly over the whole SSD, so the wear level is approximately the same for all memory cells during drive’s lifetime. Assumption about wear–size relation comes from premise that larger drive will probably contain more unallocated space which could be used for wear leveling. Needless to say that this does not hold universally.

Trim command on the other hand is issued by operating system, which notifies SSD which pages no longer contain valid data, and recent versions of both Windows and OS X supports it (although Apple does not enable it by default on 3ʳᵈ party SSD, so one has to use Trim enabler or similar application to get it working.

To answer your questions:


No, wear leveling does not care about partition scheme. Yes, Trim will work correctly.
Windows does memory caching of drive operations, yet it happens in RAM and does not involve other drives.