I have a 2017 27" Retina 5K iMac with a 4.2 GHz i7 processor, 16GB of memory, and a 2T fusion disk, currently running Catalina. I would like to make things “snappier”, especially when opening applications. I’m not that much worried about boot speed, as I reboot at most once a week or less. Would it be more bang for the buck to double the memory to 32GB (approx. cost $95) or use an external Thunderbolt 3 2T SSD as the boot drive (approx. cost $500), assuming that one had to choose rather than do both? Thanks for any recommendations.
Let me point out just 2 things. First, the TB SSD will remain useful even when you move to a new Mac. The RAM is basically only useful as long as you stick with that 2017 iMac. Second, you can get a serious 2TB TB-connected SSD for substantially less than $500. In fact, with the $100 you save there you can go ahead and buy that RAM too. 
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@Simon Thanks! For 2T, would you go with the Samsung 2T version of that same SSD ($300), or recommend a different one?
I completely concur. My 2019 iMac has only a 256GB SSD…essentially because I was a moron and didn’t think it through properly when I bought it. I received a 2 TB TB3 SSD Envoy Pro FX on Sunday and installed Big Sur on it followed by Migration Assistant to move users, apps, settings and everything else. My primary reason was to have my Adobe Lightroom catalog on an SSD for speed…but as an observed phenomenon this external drive is daisy chained off of my OWC ThunderBay Mini RAID where the actual photos reside…and the external is faster for both read and write than the original Apple SSD. I was encouraged to do this upgrade by Glenn’s recent article and it was far easier than I expected…in fact the hardest part will be reconfiguring all my backup stuff once I move the Lightroom catalog to the SSD.
I would have no hesitation to go with the 2TB Samsung. Those are excellent SSDs.
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Just to chime in, @Simon is exactly right. The SSD is the most important, and if you save some money on the SSD, you can get the RAM too. It won’t make as much of a difference, but it could help at times.
Not quite your scenario but about three years ago I replaced a spinning drive in a 2013 27" iMac with a 1TB SSD. It involved opening the iMac which is another story but that upgrade has me still satisfactorily using that Mac as my main machine when I’d been ready to dump it. I have played with booting from other external SSDs and even with USB that’s been quite a feasible option. An SSD has accelerated numerous fundamental processes on my Mac whereas previous RAM upgrades have had less obvious impact. 16GB isn’t a trivial amount of RAM so I’d doubt you’d see much everyday impact from doubling it. (Subject to your specific usage, of course.) That sounds like a vote for the SSD!!
Thanks for your comments, I’ve got my external SSD on order! I’m currently running Catalina; rather than do a clone then upgrade to Big Sur, I think I’m going to do a clean Big Sur installation followed by Migration Assistant from the internal drive. It’s been since 2017 (when I got the iMac) when I last started from a clean install and migration.
The internal drive died. Used its clone for about 6 months until the M1 arrived
Got a 2 TB machine.
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Just a followup - I decided to splurge and got an OWC Envoy Pro FX 2T SSD. I did a clean install of Big Sur onto the SSD, and when it rebooted after installation, I ran Migration Assistant to bring everything over from the internal Fusion drive. This was by far the longest part of the job. Now I’m up with Big Sur running on the SSD and everything is indeed much faster, in some cases it seems faster than my 2020 MacBook Pro. And I haven’t run into any real issues with moving from Catalina to Big Sur, although it’ll take some getting used to visually.
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