So I just finished copying and organizing all data to my new 2TB hard disk and now I am ready to nuke and rebuild on both my iMac and MacBook Pro. Both these machines can run all OS's from 10.5 Leopard through 10.10 Yosemite. Should I choose Mountain Lion or should I choose Yosemite? Mountain Lion has support for older apps. I know that iCloud Drive is not supported on ML, but if you 'upgrade' to iCloud Drive, does this mean you cannot use your entire iCloud account with ML? Are there any new apps not supported on ML? Does Airdrop work between ML and YM? Are there any other incompatibilities between ML Macs and YM Macs? Will ML be faster than YM on these 2009 machines (both have 4GB RAM)? If I install Yosemite then I cannot go back to ML, but if I install ML I can go to YM. But I've heard that upgrading the OS causes gunk and stuff... which is what I am getting rid of with fresh install. Anything else I should consider?