For me, normal lifespan is too short since a sim can't even experience an entire year of seasons on that lifespan, and long lifespan is a bit too long. However, long lifespan is good for DU so that the student doesn't waste their entire young adulthood at university. I'm a rotational player so I only allow my active household to age that way I don't miss out on important milestones with other sims while I'm sticking playing one sim through to graduation at uni. I use the long lifespan and manually age my sims up when enough time has passed, somewhere in the middle of a traditional normal lifespan and long lifespan timeframe.