If it's not too late, I'd recommend neither, at least for now.
I put PSE 13 on a brand new 5K iMac and it is not usable. After a few operations the editor hangs for several seconds before doing anything. There are many, many postings in both Apple and Adobe forums about 5K iMacs slowing down when using various programs, including Mission Control, PSE, Creative Suite, and Lightroom. Apparently Apple is aware of at least some of this and there is evidence that they are working on it (but no generally accepted resolution that I have found as of OS X 10.10.2). It may be a combination of issues with Yosemite and with the large number of pixels to generate on the 5K iMac.
Adobe (with their usual awful customer support) hasn't acknowledged anything and may or may not be working on it. If I knew that PSE would be unusable (and that Adobe offers no support whatsoever for it) I would not have spent the extra money on a 5K iMac, at least until they fixed these issues.
My advice is to wait for a confirmed fix from Apple to the 5K iMac/Yosemite issues or be willing to find another photo program on it. (By the way, the PSE editor by itself - no PSE organizer running - works fine for me. Of course, that's not acceptable.)