In my experience SSDs are great for general system performance, but I'm not sure it would help preview mode. Preview tends to just read a bunch of files from disk. If you have lots of RAM and a modern OS, it will probably keep a cache of regularly-used files in RAM. So while the first few previews might be slow, eventually it loads the regularly used files in to RAM then it's not hitting the hardware at all! So maybe just make sure you have a couple of GB free in RAM for the OS to cache with.
As
saiyadjin says the strong point of SSDs is random read/writes which is things like starting up a complex app, or booting Windows.