Short because I tend to get super bored fast with longer lifespans (although that's a lot better now since I started to create a story of my Sims' life), but since short lifespan got a lot longer anyway with the recent changes and the addition of infants it's fine for me. I also like to have a challenge regarding my Sims' money, skills and career and don't want all of them to max everything super easily so that's also a reason.
I used a mod to shorten pregnancy time for a while (2 days instead of 3), but I stopped using it because I didn't like that NPCs got even more children this way (I enjoy Neighborhood Stories but it was just a little too much).