actually, "I give you something of mine and you give me something of yours" is called trading, not sharing - sharing is "I'll let you have my files"
the cache choice doesn't effect the caches used any more, the new caches detect your region automatically by your IP Address, however the network is not made up of different regions all on separate networks, it is 1 big network with every user on it - any user can find files from any other user on winmx. The regionalisation for the caches is done merely to help get the network arranged better (so Japanese secondaries connect through Japanese primaries, and European secondaries connect through European primaries etc) for more efficient routing of data.