Should i take vitamin b12 or vitamin b complex?

It depends on: if you have any nutritional deficiencies, reasons for a nutritionally incomplete diet or malabsorption syndromes. Remember if you don't have any defined nutritional deficiencies you are better off getting your micronutrients like the B vitamins from whole foods rather than from supplementation. Alternatively if all you have is a B12 deficiency then you should be fine with a B12 supplement.