The most important thing, in my opinion, is to adopt a notation that is consistent with whatever is most common in the broader culture. Geometry notation is highly context-specific; notation used at the secondary level tends to be different from that used at the undergraduate level, and there are country-to-country differences as well. If there is any kind of national or regional school-leavers exam (comparable to A-levels in the UK or to the SAT in the United States) you should use whatever notation is standard on that exam.
I don't know much about the Swiss educational system, but I find it frankly hard to believe that there is no standard notation in use in your context. The Swiss geometry curriculum was one of nine national curricula compared in:
Hoyles, C., Foxman, D. and Küchemann, D. (2002) A comparative study of geometry curricula. Qualifications and Curriculum Authority, London. ISBN 1858385091
...and while I don't have access to that book at the moment the mere fact that it exists suggests to me that there is a national geometry curriculum in Switzerland.
If you genuinely do teach in a vacuum, free of external encumbrances and with no cultural norms to align yourself with, then I guess you are free to choose whatever makes sense to you.
In the United States, standard notation at the secondary level is:
- $\overline{AB}$ denotes the line segment from $A$ to $B$
- $\overrightarrow{AB}$ denotes the ray with initial point $A$ and passing through $B$ (but note that a conventional Geometry class in the United States typically does not include vectors, so there is no conflict with that notation)
- $AB$ denotes the length of the segment $\overline{AB}$
- $\overleftrightarrow{AB}$ denotes the line through $A$ and $B$
At the advanced undergraduate or graduate level, different notations tend to be used; for example, I believe Greenberg's text uses $AB$ to denote a line, rather than the length of a segment, but otherwise uses the conventions above.
