As mentioned in the article, How To Make Your Own Window Sill, you do NOT need to buy one to measure angles. The article mentioned several “manual” methods to accomplishing the same goal.
However, having a small inclinometer comes in handy when the space is tight.
Also, if you purchase one with a magnets, you can expand the use such as adjusting the angle of a blade in the table saw.
I like this Johnson Level and Tool Angle Locator because it has no batteries to worry about and is sturdy.
It even comes with a magnet to attach to metal surfaces. The only drawback is that the numbers on the gauge is quite small.
If you have tough times reading small number, I recommend a Wixey Digital Angle Gauge Type 2 with Magnetic Base and Backlight.
It is very compact so it can work in some tight spaces (like measuring a window sill angle WITHOUT removing the exterior storm window frame).
In addition, the unit has magnets that can attach to metal surfaces like table saw blades, etc. so that it can be used in lots of different situations.
The only drawback is that it has delicate electronic components inside so DO NOT DROP IT.
At the end of the day, either unit will do the job!