For photography, it is not that important depending on the level/quantity of photography you process. Video is critical, unless you like long - go have dinner - rendering time. That said, I wouldn't build a PC without dedicated memory. Even my futile attempt at a hackintosh (tried to put a spare SSD to use and dual boot) had dedicated memory. Most critical are SSD and total system memory. Even an i5 is sufficient for photography with i7 preferable and mandatory for video. i9 is overkill - save your money. Lightroom is a memory hog so the more system memory the better. Even with a dedicated 'gaming level' memory card I still have loading lag of large RAW files initially appearing out of focus.
While I use a PC desktop as my preference for photo processing which I personally built several years ago, (Core i7 4.0Ghz, 512 GB SSD + 4TB hard drive) I've never upgraded memory to motherboard max, running 16GB. However is paired with a Nvidia GTX with 4 GB memory. Upgrading to a Mini vs iMac has been a quandry, but ruled out the previous Mini as insufficient for FCPX. Haven't researched the new one, but lack of dedicated memory could be critical for FCPX.
While I don't like to do photo processing on the notebook, I obviously can only use FCPX on the MBP and is part of my dump Adobe campaign. As such, it wasn't your basic (late 2013) MBP. I think I left one option off - which ironically was dedicated memory, but is Intel Core Graphics 1536 MB memory paired with a 2.3 ghz core i7 and 16 GB memory. I really don't have any problem running Lightroom or FCPX with that configuration. Not surprising, as looking at a CNET review of the 2014 MBP with the same Intel graphics and the compared it to their prior reviews, including a 2013 MBP with the Nvidia dedicated memory, and the graphic performance was almost identical on the two MBPs.