Main memory (RAM):
- is faster
- is very low latency
- is more reliable
- is more expensive
Swap is just virtual RAM on disk, and as such, it inherits the usual pros and cons of storage drives, depending on which type of drive it is. It is used for inactive pages when the system needs more real RAM.
It is recommended to have swap space available in case the system fills all the RAM, to avoid hiccups or a freeze. However I don't think it's possible to move processes by hand. The Linux kernel is written to handle memory pages in an efficient way and keep in RAM what it needs and swap what it doesn't, so you shouldn't need to do anything on your side.
You can adjust the swappiness as per the link you posted; that value determines the percentage of RAM that must be full before the kernel starts swapping.