Most routines get complicated long before they get effective. Three things do the bulk of the work, and the question is rarely which to buy — it is when to put them on.
Morning: vitamin C, then sunscreen
Vitamin C is an antioxidant. It works against the damage that daylight and city air do while you are out in them, which is why it belongs at the start of the day rather than the end. Sunscreen goes on last, over everything else, because it has to sit on the surface to do its job.
The two are a pair rather than alternatives. Vitamin C handles what gets through; sunscreen reduces how much gets through in the first place.
Evening: retinol
Retinol is a form of vitamin A. It asks the skin to renew itself faster, and that request takes a few weeks to be answered. It goes on at night, on clean dry skin, and it does not need company — a moisturiser afterwards is usually enough.
It also makes skin more sensitive to sun in the weeks you are getting used to it, which is the second reason sunscreen is not optional.
The rule
Antioxidants in the morning, renewal at night, protection over the top. Medik8 built a whole philosophy on exactly this and called it CSA — vitamin C, sunscreen, vitamin A — and it is a good shape for a routine whatever brand you use.
What not to do
Do not start all three in the same week. Add one, give it a fortnight, then add the next. If something stings, that is information, not a stage to push through.
And if you are unsure where to begin, begin with sunscreen. It is the only one of the three that works from the first day.