Honestly it depends on your needs. For example, on a normal day where I have few programs open I use 7GB. on a work day where I have all my programming software running and few other software I peek around 13GB.
for me 16GB is prefect for now. 8GB is not enough for me at all.
So look at your needs. do u need more RAM? getting more ram will not increase performance unless your using all 8GB.