Lactic acid or glycolic acid: which should I choose?

Lactic acid is the gentler of the two and helps skin hold on to moisture, so it suits dry and sensitive skin. Glycolic acid exfoliates more strongly and suits normal, oily and less sensitive skin.

Which lactic acid strength should I choose?

Start with 5%, such as The Ordinary Lactic Acid 5% + HA 2%. Move up to 10% once your skin is used to it.

Is lactic acid good for sensitive skin?

It is one of the gentlest exfoliating acids, so it is often the best acid to try first. Start with a low strength and patch test, and choose a different product if your skin feels irritated.