In the following article, scroll down to the “microSD” section:
Even with a high-end micro SD card, don’t expect better that ~20 Mbps writes. Fortunately, for garden-variety home-automation purposes (intermittently writing logs), that’s adequate.
The principal advantage of using an SSD (as opposed to an SD or USB thumb-drive) is superior longevity. However, don’t expect them to be a miracle cure for all illnesses. If the RPi’s power supply is marginal or power is interrupted during disk-writes, the SSD won’t magically fix these errors.
Don’t bother buying a high-performance SSD for the RPi; you won’t experience the extra performance. A low-end SSD is capable of operating faster than what the shared USB 2.0 bus can sustain.