0.2mm - These needles cause just about no side effects and you can barely even feel it against your skin. They are meant to treat extremely small problems such as dull skin. These products are only to make your skin look brighter and stronger. They are not intended for collagen induction, but good for improving creams & serum absorption to the skin and is also suitable for daily use. Recommended for young, sensitive and thin skin, e.g around eye area.
0.5mm - This is recommend for best results. It is perfect for facial skin anti-aging, fine lines, wrinkles, scarring & hyper-pigmentation. These needles will increase the collagen growth and increase the effects of products you want to use by making absorption easier. These needles will be more painful than the 0.25mm ones but it is not unbearable by a long shot.
If you are unsure but would like to try it out, you can start treatment with the 0,2mm needle length and see how it feels. Once you feel comfortable, change to 0,5mm for maximum results.