I can have either a regular HDD or pay a bit mor and have a SSHD at the same size n my new laptop but what is the difference? Both drives are 6GB/s 5,400RPM.
The SSHD you are talking about is probably what is also referred to as a hybrid HDD. It contains a small SSD built-in to the drive to act as a persistent cache (not to be confused with the cache buffer all HDDs have). The purpose of the small SSD is to store most used access to flash memory so booting is quicker and some often used pgms load quicker. But at only 5400 RPM, they are probably intended for laptops, not desktops which usually have 7200 RPM drives.
sshd basically gves you ssd-like bootup speeds plus accelerating a few more frequently used apps.
There's typically only 8gb of SSD cache on them and for the price difference I don't find them worth it.