I think the answers to the OP have got it covered - RAM is fast, volatile and not cycle limited, NAND is less fast, non volatile and wears out. BUT I'd like to mention another angle on this that has been in my mind for years.
A mechanical disk is a sequential R/W write device, that is, you start reading or writing at a particular spot, and keep going along the stream of bits until another spot where you stop. If you want to swap locations, you need to navigate the R/W head to the place where the new data is. Memory, by its nature, is random access - you can read/write each cycle to a completely different place in memory with no performance penalty.
Now, current SSDs are engineered to look like sequential R/W devices, simply because that's the known paradigm they are replacing. But they're really not sequential - they are memory. So what if we had a new kind of interface that allowed memory-style random access to the memory in an SSD, while being able to distinguish it from RAM.
I'm mainly thinking of databases here - they are traditionally based on disk, but the bottleneck has always been the sequential media - in fact, a lot of database servers actually have a map relating data directly to disk sectors in order to increase speed. If we could create a database server that mapped data directly to random access SSD-style nonvolatile memory, it could be a game changer for database speed.
Perhaps I'm dreaming and the SSD interface approaches the speed of direct random access, since that's what it's doing internally (plus it offers other services such as write cycle distribution).
I've just never heard this concept stated directly before, so I thought it was worth mentioning.
(edit: whoops, I see @psusi already mentioned this. I'll leave the answer here as an elaboration anyway)