for SC189 it would make sense to use it for higher currents (like 0.5-1A) as that's what it's optimized for.
You can look in datasheet -
https://www.semtech.com/uploads/documents/sc189.pdf - at page 5 and you see there two charts on the right side.
1. Efficiency vs Load Current (Vout = 3.3v) with a curve for 4.0v and a blue curve for 5v
2. Efficiency vs Load Current (Vin 5v , V out = 3.3v) with multiple curves depending on inductor chosen
So you can see in first chart that with 4v (what would be typical for a lithium battery, let's ignore the 5v curve on that chart) , with the recommended inductor (or one with same properties) they think you're gonna get around 90% efficiency at 0.1A and peaks at round 0.3A ... 0.6A at around 93-94%and then above 0.6A the efficiency will slowly go down towards 90%
Below around 0.1A, the efficiency goes down significantly, but it's still gonna be around 75% at let's say 25mA (the smallest tick on the x asis is 0.1A but let's estimate)
So let's say your lithium battery's voltage right now is 4v and let's compare this regulator to linear regulator at these loads :
The linear regulator's efficient is same across the whole range... voltage goes in, voltage goes out .. same amount of current + tiny current consumption of ldo.
Vin ... 100%
Vout = ? % => ? % = Vout x 100 / Vin = Vout x 100/4 = 25 x Vin = 82.5%
The switching regulator will be 75% at 25mA, 90% at 0.1 , ~93% at 0.3..0.6A , ~90% at 1.5A
So the linear regulator will only be more efficient at super low loads.
If the battery voltage changes a bit, let's say it goes down to 3.6v .. the linear regulator becomes a bit more efficient efficiency is : 3.3v x 100 / 3.6 = ~91.6%
The switching regulator may still be more efficient at 0.3..0.6A range, but be less efficient outside this range.
However, keep in mind that the linear regulator will have a minimum voltage drop. For example, the linear regulator may have 0.1v drop at 0.1A, 0.25v at 0.3A and 0.4v at 1A or more.
In this case, the regulator would output 3.3v at 0.1A, and will output 3.3v at 0.3A (because 3.6v - 0.25v = 3.35v) but at 1A, the linear regulator will only output 3.2v (because 3.6v - 0.4v = 3.2v)
So at 3.6v, the linear regulator may become unusable.
But let's look at that Torex regulator with a maximum 200mA output...
See datasheet -
https://www.torexsemi.com/file/xc9265/XC9265.pdf -
Right on the first page, you will see a chart for the version of the chip with 1.8v output and you can see the curves for 2.7v, 3.6v and 4.2v - you an estimate similar curves for 3.3v output version
But what's cool in that chart is that even at 0.1 mA the efficiency of the chip is already at over around 85-90% and stays relatively flat up to 100mA or so.
So you could use this regulator for 3.3v < 100mA, and switch to the SC189 regulator from 100mA and higher.
You'll have 85-90% efficiency below 100mA, and then you'd have 90% to 93-94% up to around 0.6A and then slowly go down towards 90% efficiency.
A switching regulator will also be able to work with much lower input-output differential (linear regulators have to deal with that voltage drop on the internal transistor)
Even at 1mA sleep power consumption, a linear regulator will be as low as ~78% efficient, while this Torex IC will be around 85-90% efficient, if they don't lie in datasheet. That extra 10% efficient is not something to sneeze at.