In the riser if your staircase is closed, in the wall if it is open or if you would rather not open the riser up. Functionally they do the same thing, but the wiring runs differently and that decides the choice more often than taste does.
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What the riser is
The riser is the vertical face between two treads, the part you can stub your shoe against. If it is not there, you have an open staircase and this option falls away immediately.
In a closed staircase you drill a hole at about a third of the tread width, slide the fixture in and let the light fall straight down onto the tread below.
Why the riser gives the best light
The light comes from exactly the right side. It falls on the tread your foot is heading for, from above and from close by, without you standing in between.
You also barely see the fixture. The cover with light below protrudes 10 mm and the light source stays hidden behind the rim, so from below you never look straight into it.
The drawback is the work. Wiring has to run behind the riser, and in an existing closed staircase that often means the riser has to come off or you have to work from underneath the stairs. On a new or renovated staircase that is no problem; on an existing one it is.
Why the wall is more often the practical choice
In the wall beside the staircase the wiring sits in a chase or in a stud wall, and during a renovation that is usually easier to arrange than inside the stair timber.
The height is then a choice you make yourself. Too high and you are looking at the light source; too low and the light does not reach the tread. A usable rule of thumb is 15 to 25 centimetres above the tread the fixture belongs to, so low enough to stay out of your field of view.
The further the light point sits from the tread, the tighter the beam has to be. The cover with a 60 degree beam is made for exactly that: the light arrives concentrated rather than fading out against the wall.
What you need to know in both cases
The cut-out size. The Trevo base unit needs Ø46.5 mm. The Steno needs 52 mm and 34 mm of recess depth. That last figure counts as soon as you drill into a thin riser or a stud wall.
The space behind. You need room behind the hole for the fixture and the through connection. In a riser that is the hollow space under the tread; in a wall it is the chase or the cavity.
Everything in line. Mark the positions out before you drill and measure from one fixed side, not from the tread itself. Stair treads are rarely exactly the same width, and a row of light points that does not line up is something you always see afterwards. The same rule applies to spots, see How far apart should ceiling spotlights be?
The advantage of 230 volts
In both positions wiring runs from point to point. If the module works on 12 volts, a transformer joins it and you have to work out how many points fit on it.
The Trevo runs directly on 230 volts, so you simply loop through. That helps most in a wall, where you would rather cut one chase than find room for two kinds of wiring.
And if you have already drilled
The cover slides off the base unit without tools. Once the lighting is in and you do not like the look, you change only the cover: from square with light below to round, or to a version that lets more or less light through. The drilled hole stays as it is.
How many points you need is in How much stair lighting do you need?, and if you have an open staircase, different rules apply: see Stair lighting on an open staircase.
The whole system is at stair lighting, and how it is built up in How does stair lighting work?
At what height do you fit stair lighting in the wall?
About 15 to 25 centimetres above the tread the light belongs to. Higher and you are looking at the light source, lower and the light does not properly reach the tread.
Which cut-out size do I need for stair lighting?
The Trevo base unit needs Ø46.5 mm. The Steno with motion sensor needs 52 mm and 34 mm of recess depth. Take those figures straight from the product data.
Can stair lighting go into an existing closed staircase?
Yes, but wiring has to run behind the riser. If you cannot reach the underside of the stairs, lighting in the wall beside them is usually simpler.