The Western Digital drive you chose to compare with is a fairly weak example of what SSDs are capable of - it's likely an older generation SSD. Modern SSDs, with the most popular example being being the
Samsung X5, shows the difference more clearly. Your Sandisk USB drive talks about read/write speeds of around 300-400 MB/s, but the X5 has read/write speeds between 2,000-3,000 MB/S... almost ten times faster. As ruslan120 also alluded to, the random read/write speeds (which generally matter more than pure transfer speed) tend to be much faster on dedicated SSDs than on USB flash drives. Unfortunately, despite their being the numbers that matter more, they don't tend to be advertised or as readily available.
The type of flash memory - which impacts overall drive longevity (how many read/write cycles they can go through), and drive speed - is likely the higher-end architecture in SSDs, although this is also not advertised as heavily as it once was. Lastly, most USB drives are made for copying files to them and then copying off; they're not really designed for frequent access, as might occur with some photo libraries (depending on what program you're using). USB drives I've used in the past tended to become pretty warm with extended usage, which makes me think they'd be unstable for frequent operations and heavy usage; not sure if newer drives have rectified that.
Long story short, if you're low on space and need something that has fast access times, get a real SSD. If you're low on space and need something cheaper but with decent access times, get a standard HDD. I don't trust USB drives for anything more than shuffling files around.
Regarding online backup solutions, you have a few options. Apple has a built-in solution specifically for Photos that backs up everything, and that would only require that you buy more iCloud storage space if your library doesn't fit your current space. In theory, even if you don't use Photos or don't want to do it through Photos you could move your photo library to your iCloud directory for the same effect, if you want to stick with iCloud.
In the realm of personal, online backup services, I use
Backblaze. If you're only looking to back up one computer, and particularly if you have a lot of data, Backblaze is the ideal solution. Compared with other backup services, there are no storage limitations; it's also one of the few services to offer a native Mac application that runs pretty seamlessly and stays out of your way. When I had priced them out a few months ago, Backblaze was also one of the more affordable options. If you're looking to back up multiple computers and/or if your storage needs aren't that high, there may be better solutions. Backblaze literally only works on one computer at a time, and there is no "family" plan option. For me, I have my photo library, along with family videos and other data, on a Drobo that takes up a few terabytes... Backblaze has the entire thing backed up, which is wonderful peace of mind.
As a note, the online storage services aren't really meant to function like Time Machine. Some do keep versions for a while, but Backblaze basically mirrors your computer and external drives; if you delete something and Backblaze performs its sync, I don't think you can get the file back through their services. But if your computer or a hard drive dies, you can download your data from them when you have replacement space available and/or ask them to ship you a hard drive with your data.
Plus, Backblaze is just a really cool company. If you've ever read some of those reports about which hard drives have higher failure rates than others, there's a good chance that Backblaze published those figures based on their data farm's hard drives. I like to read their newsletters, and the overall feel is one of a small team that wants to do right by their customers. They even had a price hike in the recent past, which still kept their prices very reasonable... based on the comments, people were saying that the company could have raised prices even more if they needed to, and they'd stick with them. I feel similarly: I want them to stay in business, as they're offering great value (even with the mild price hike) and seem like they genuinely care.
