So I have an aging 2009 iMac 24"...What I am stuck on is how much of an advantage is the 512GB or 1TB Flash would be over the 3TB Fusion Drive when working with iPhoto or iMovie?...My concern with 512GB is that it is not enough to support the long term increase of my iPhoto library size (already at 250GB)....Base Model 5k Retina with 1TB Flash: $3299....I will likely keep this iMac for 5-6 years...
I doubt you'd generally see a major performance difference between SSD and FD for iMovie or iPhoto. The issue is not how fast the I/O is, but whether it's the slowest link, in conjunction with CPU and GPU. Once above a certain threshold, further speed I/O speed increases won't make overall application performance any faster.
In either case, whether SSD or FD, a 2014 iMac would be hugely faster than your current machine. So you'll be very happy no matter what you get, the question is how to best optimize the configuration for your current and projected workflow, and within your budget.
Re long term growth over 5-6 years, if you shoot raw stills (or may ever do this in the future) this takes a huge amount of storage. Each raw photo from a Nikon D800 is 40 megabytes. My small photo crew often shoots 100 gigabytes per *day* of raw stills.
While it seems you'd want to sacrifice everything for faster I/O, there is a cost and penalty to each decision. If you watch Activity Monitor or iStat Menus while LightRoom is building 1:1 previews on a big import, it is largely CPU-bound, not I/O bound. An infinitely fast SSD wouldn't make it much faster. It's the same with FCP X for many operations.
You want fast I/O -- but only to the point where that majorly benefits your *application workflow*. Beyond that, you're paying money to produce higher benchmark numbers, which means you're *not* paying for other beneficial improvements.
If you have no financial limit, just get the highest-end retina iMac with 1TB SSD and a big Thunderbolt SSD RAID.
If you do have financial limits, you must consider tradeoffs. E.g, your above-listed base 5K retina iMac with 1TB SSD is $3299. For less than that you can get a similar retina iMac with 3TB FD, *plus* 4Ghz i7, *plus* 4GB M295X GPU. In most cases I'd argue the latter machine is a better long-term match for your stated work. You can always upgrade the memory later.
Another option is getting a 256GB SSD retina base iMac and a lower-cost Thunderbolt RAID drive for all your data. That would be less than a 1TB SSD iMac, but it would lack the 4Ghz i7 CPU and the M295X GPU, both of which cannot be upgraded.
A 256GB SSD retina iMac with 4Ghz CPU and M295X is $2,999, so that plus a 4TB GTech GRaid would be about $3,450, total. However that would be RAID 0 so you'd definitely want that backed up. But really everything should be backed up whether SSD internal or RAID 5 external.
