Caveats and which upgrade path from Mountain Lion My 2010 i7 iMac running off an internal SSD is currently on OS X 10.8.5. I need to upgrade it to at least 10.9 to use a certain software package because they stuck fixes for a 10.8 compatible version in with a feature that necessitated a higher OS version!

My question is this:

I have a simple choice between 10.9.5 and 10.10.2 are there an major issues/gotchas with either of these versions that I should take into consideration? Privacy? Network issues? Compatibility issues?

The trouble with all the searches I have done so far is that they mainly turn up puff-piece reviews by kool-aid drinkers that evangelise any and every upgrade and cloud integration whereas I want the warts and all info. 

I have noticed the Spotlight sends queries to Apple and Bing which can be switched off.
The continuity/handoff stuff isn't applicable as the iMac is pre-BT4.0.

Is there anything privacy invading or regime/doctrine changing between Mavericks and Yosemite that may lead me to choose the former over the latter rather than just use the latest? 

I am in the process of testing with each but, as you can guess, that is a highly involved and long-winded process that will likely leave some stone or other unturned.
So, I have performed my testing and taken the leap of faith albeit with a disk image to roll back to. I performed the update by way of a fresh install and data restore from Time Machine.

iMovie 10.0 is somewhat problematic. It offers the ability to read in my XAVC files but simply refuses to convert my iMovie v9 events. It'll do a handful then leave the others without mention of why. This seems a common issue. I have opted instead to use Media Converter - a free application - with a preset (Re-wrap AVCHD for Quicktime - uncompressed Audio) to enable iMovie v9 to utilise the files. I also do not like the way the new iMovie utilises an iPhoto-esque hide inside way of dealing with movies. That it doesn't convert existing events just ices the cake. My preference is for how iMovie v9, Lightroom etc work.

With regards other software I have had to switch from Zevo (no longer maintained) to Open ZFS. The latter doesn't auto-mount the drives but the switch is a blessing in disguise. It is actively maintained and utilised less CPU for the same work.
Parallels is fine as long as you've updated to the latest version. MS Office is all good. The iLife and iWork suites update but leave behind the previous versions which is a nice touch. I am yet to find any issues. I am not a major user of iCloud other than appointments/reminders which is all good.

Please feel free to tag onto this posting with notes of issues you have found. iMovie is my only item of note.