[Dudi]

1.because RAM is a volitaile storage . it can store much faster but it lost all the data when the power is cut . ssd on the other hand is a non volitaile storage that mean if you cut power your data will be still intact .

2.ssd mostly use MLC (multi level chip). MLC is much faster and reliable than TLC (third level chip) that used in flash drive . not only that but ssd have many NAND chip and flashdrive only have 1 or 2 NAND chip . and also ssd use SATA or M.2 flashdrive use USB

Nowadays only very expensive SSDs use MLC because TLC is much cheaper.
I really like Sandisk solution they have about 10% SLC chip for most used files and 90% TLC for less popular files, this way only the SLC chip rewrites mostly. 
From second hand it's avaible for about 100 dollars the SanDisk  Ultra II 480GB. I think nowadays this is the best buy.