Hi Mr. Navigator,
A 5 niter is not a bad way to start into the world of cruising. If time off from work/school and money are an issue then a 5 niter would work out fine. However don't rule out the 7 niter either. 5 nites does go pretty fast! My first cruise I went right for the 7 niter.
I'm sure you will enjoy your 1st cruise if it is a 5 niter or 7 niter!