The raw speed benefits of SSD are mostly lost on the Pi computers, due to their single USB 2.0 interface. However, SSD typically has much better random IO performance (IOPS), so they can speed things up slightly when compared to micro SD cards. It's not going to be a huge speed boost, so caching anything won't accomplish much (and may even be slower).
If your storage needs are modest (<=64GB) just get a good "A1" rated micro SD card. I have several Pi computers running off of SanDisk Ultra A1 cards, and I've been happy with their performance. In sizes of 128GB+ an SSD makes more sense, because large micro SD cards get expensive (more expensive than SSD at 256GB+ sizes).
Data integrity should be better with SSD, assuming you're not getting a crap SSD from ebay or Alibaba. If you are only using the SSD as storage (and booting from a micro SD card), then just about any USB SSD or USB-SATA adapter should work. If you want to boot and run the OS from SSD, well then things can get a bit tricky. The Pi boot-loader is not compatible with all USB mass storage devices, and finding something that works reliably often requires some trial and error (and wasted money). There's lots of info here on the forums regarding booting from SSD. If you need more help with that, try a search, and ask if you can't find what you need.
As far as SSD vs HDD goes, in my experience HDD is even slower than micro SD card, and much slower than SSD. They are still the king of storage vs price, and even better for data integrity, but if faster performance is your goal, HDD is not the solution. USB boot from HDD is also more troublesome, due to power requirements and slow spin-up times for HDD. Not saying it can't be done, but it may require even more trial and error troubleshooting than SSD (simpler to just use HDD as storage device).
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?