If you plan to keep using your new iMac for the next 5 to 7 years, which is NOT unusual, you will not want to be stuck with and OS that is at least 8 years old!
Yosemite has been around for at least a year and El Capitan is almost ready for release. Given your apparent anxiety and reluctance to upgrade, why not wait for a little while after El Capitan is released and install it after the first update to it?
I'll presume that you will continue to update the Adobe Apps as the years go by and perhaps one of those updates will REQUIRE El Capitan or later, leaving you with another choice.
Bottom line, wait for the first version upgrade to OS X 10.11 and then install. Meanwhile, ALWAYS install ALL the updates that Apple throws down at you. They are specific to your machine and the software that you are running.
P
I don't know whether I should update my iMac to El Capitan or not. I'm planning to use it daily as my primary computer for the next 5 to 7 years (yes, seriously) and too afraid to update to the next OS because I think it will not be as fast as it is now. I value speed and efficiency much more than the cool new features. My question is: will updating my iMac to the new OS every year decrease its performance over time or not?
iMac is used mostly for web browsing and Adobe apps (Photoshop, Illustrator, Premiere Pro, After Effects, Lightroom; in that order). I have very few files on my iMac because I keep everything in the cloud (~150 GB/1 TB) and there are ~25 programs installed.
iMac (Retina 5K, 27-inch, Late 2014)
OS X Yosemite 10.10.5 (latest)
4 GHz Intel Core i7
16 GB 1600 MHz DDR3 (will upgrade to 32 GB in the near future)
AMD Radeon R9 M290X 2048 MB
1.12 TB Fusion Drive
Thanks.

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