Purists would choose either depending which suited their wrist best. The fact that FPJ has discontinued the 38mm case size is mainly due to the poor demand for this size according to their sales director who I was fortunate to meet recently. I was quite surprised by this comment as I would have thought that 38mm would be more widely received. Perhaps it is because FPJ is more popular in the USA and Europe, I am unsure.
One point worth noting is that although the case size may have increased, the movement has not so I am always partial to a movement that fills the back of the case more completely. In this aspect, the 38mm case triumphs.