a emmc does around 220mbs read and write, i don't remember, is like half of a ssd
a hard disk can't do over 160 read constantly on most models
if you are seeing laptops, remember that emmc comes soldered in the laptop so what you get is what always will be
some models using emmc doesn't have a sata port, so when you get bored of the 32 or 64 gbs of space on the emmc, you won't be able to get more space having a hard disk in there