I always sew first and then serge. I only keep white or black thread on my serger, so sewing first with matching thread keeps the thread from ever showing in the seam. Plus, I never 'lose' my seam allowance on my sewing machine. Serging is easy-peasy after that!
p.s. - Sewing first also gives that whole 'professional' look that most people look for, and it gives a very durable finish too since the threads from just serging can stretch a bit over time and you'll see the threads pulling in the seams.