What is the difference between the latest stable and the latest nightly versions?
The releases are made at point when most know errors were ironed out. Nightlies could countain very immature features.
If you just want to play the game, go for the stable. If you want to be involved in testing and bug reporting (if you do, that would be very welcome), use the nightly, but be sure to make regular backups of your saved games.
Although it has happened that nightlies were actually more stable than a release (100.0 was no good for example...)
The stabel are make for "Normal user"
When a Coder take a new funktion in simutrans, sometimes he/she must work on different parts of the code. So he/she could make a mistake and build a broken safegame.
For this are the nightlys. If you use a nightly, It could be, that your simutrans-safegames are after .. damaged. But if you use a nightly, you could give the coer feedback.
So the nightlys are for "advanged user" and tester
See http://en.wikipedia.org/wiki/Nightly_build (http://en.wikipedia.org/wiki/Nightly_build)
Hmm, the problem with that analysis is that there is no formal beta testing of the "stable" versions in Simutrans, so they might be as bad or worse than any of the nightlies in terms of stability...
The nightlies are for beta testing. Because any bug fix can cause new errors, you cannot beta test a stable. Rather define a beta at some point not beta any more.