Longer stem is better for rear wheel stuff, shorter for 2 wheel stuff.
If you have an echo24 like me, then the bike is really tall so you are looking for a way to balance that, the answer would be a longer stem, think of it like a mod. That's why the koxx sky comes with a longer stem than the echo 26 for example, because the koxx is higher.
Although it really comes down to personal preference and how you feel more comfortable on the bike with the stem.
Hope it helps.