Well I've decided that those upgrades are out of my budget. I'd rather have an SSD.
My 2 cents:
Drop back down to the 256GB SSD and use the savings to upgrade either the CPU or GPU (I highly recommend you upgrade the GPU) and get an external SSD.
My reasoning: External IO (particularly Thunderbolt 2, but also USB 3.0) is fast enough these days that you can get an external SSD with way more space for a lot less than Apple charges with no performance penalty (vs installing the same SSD internally). While the Apple SSDs are faster than anything except a top of the line PCIE SSD attached via an external enclosure on Thunderbolt 2, unless your working with uncompressed 4K video or some other ridiculously IO heavy task, most people would be hard pressed to tell the difference between an Apple SSD and a "low end" TLC SATA SSD (because they're both so much faster than any HDD).
Furthermore, while you can always add more storage later either externally or by physically opening your iMac (if you're advernturous) the GPU and to a lesser extent the CPU are set in stone the moment you order that machine until the end of its life. I know I might have held onto my trusty old 2011 iMac a tad longer if I'd had the foresight to spring for the high end GPU upgrade at the time as the CPU was still more than fast enough and once I added an SSD the machine still felt really fast (in 2014). Not having that faster GPU though just killed it.
I recommend the GPU over the CPU for four reasons. First, the difference between the base CPU and the high end CPU is much smaller than the difference between the base and high end GPUs. Second most quad core intel CPU's today are already very fast for the tasks your looking at. Third the trend is for more and more tasks to be pushed off to the GPU these days so investing in a powerful GPU today will, IMO, give your machine a boost down the road. Finally the use of mobile GPUs in the iMacs means that they already have a tendency to be somewhat unbalanced in terms of system resources (CPU vs GPU), mitigating this will help extend the useful life of your machine.
Anyway just some food for thought.