Resolved Yosemite or Mountain Lion for 2009 iMac?

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?

grahamperrin said:

Some of these questions are answered elsewhere …



No.

iCloud Drive FAQ – Apple Support



Yes. Here's a screenshot of five for Yosemite that are not compatible with Mavericks; they'll be incompatible with Mountain Lion.

Click to expand...

So the older iWork won't be able to work with documents made in newer iWork?

bummer... I may need that


What about speed? I am very interested in the speed.

The logitach k360 keyboard I use with the iMac doesn't work with holding keys at startup. I can't hold option key for boot menu! probably because of it's wireless shenanigans. This is the only keyboard i have D: how can I boot into recovery or boot selector?

ok i guess i will just go with YM
Cannot do it now because my logitech keyboard doesnt work at boot so I can't boot from installer. have to wait til I get new keyboard unless someone knows of a way to workaround this