The most important factor to look at is the type of wear leveling which ensures that the writes to the data blocks are equally divided over all blocks. Strategies are divided between dynamic wear-leveling where only unused blocks are considered during the reshuffling of data and static wear leveling where the algorithm may also reuse blocks which have already data written on it. Static wear leveling is superior because it uses all blocks on a drive even if the drive is nearly full, but is only used on higher end drives. Often USB sticks and SD/MicroSD cards use dynamic wear leveling while 2.5/3.5 inch SSD drives uses static.

You can find USB sticks and SD/MicroSD with static wear leveling and I use them in a number of IoT devices, but they are more expensive and somewhat hard to find. If the manufacturer doesn't mention the wear leveling strategy, you can almost be sure that it is the dynamic wear leveling which I wouldn't use for any type of random access situation. They are only useful for sequential writing procedures with a low number of erases/rewrites like in video capturing and photo storage in cameras and smartphones.

I am not allowed to give specific product recommendations here, but sticky me for recommendations.

Thanks lammert, that's very informative, and is making me rethink, depedent upon use.

An external ssd would get greater use, for sure, as I'd swap it for the external hard drive.

Prices are dropping and I'm going to investigate further.

Do these SSDs use a different architecture?

Yes. SSD's and USB memories use diff architecture, SSD will be faster, way faster. Comparing the speed between the two is not as simple as copying a file because they behave differently, one (SSD) is way better handling random access, that's why "same speed" SSD and USB memory won't behave the same way used as continuos data storage, but both can perform pretty well as backup devices (give they have the same speed).

SSD's are also built differently (you can find mainly two types of them), so it's not as simple as plain SSD vs USB memory. The question here would be what do you want this for. Backup? both can handle the job pretty well (but I would treat any of them as I treat any of my backup storage: needs redundancy).

Speed and performance? Any SSD will beat USB. Even so, the SSD will degrade over time both in speed and availability as the blocks have limited rewrite range. Depending the brand and technology you will notices this more quickly.

Anyone with experience of using these external SSDs?

I've been using both USB and SSD. USB for plain storage, backup and rarely used data, never for random access, what I mean is "drag and drop once, leave copying the files, done", copying multiple files at once suck.

SSD? love it, but I use it as data storage where no temp or swap files are used as this degrades performance. This things are great because of their speed and also due to using less energy, they are more power efficient. You might notice this while using your laptop (if that's the case) far from any outlet compared to pure HDD use.

External? limited use in my case. Worked great but later converted the HDDs to external and kept the SSD internal. In my case using a laptop was no problem because I removed the DVD and now I'm using a HDD bay, so I have two disks inside the laptop. The DVD on most computers has less speed than the official HDD connections but I honestly don't notice any difference in my case. Anyway I know eventually my SSD will have to be replaced.

Yes this things are really cheap nowadays. Using SSD's can bring old computers back to life with greater performance. If your budget allows you to use only SSD's then go for it but keep in mind the backup tasks as always.

Thanks explorador, that's helpful information on the best route for memory expansion.

I only have one laptop with internal SSD (no traditional hard drive), and it's fast, very fast: A real productivity enhancer.