After reading many posts about the best way to travel to ham tien (mui ne) l thought l would comment on our families experiences. We opted for the train from saigon to phan thiet and then a cab. The train left around 7 and 4 hours later we arrived. The trip was easy. No need to be at the station much more than 15 minutes before departure. We travelled with our 2 and 9 year old who spent the time using electronic devices or in the case of the 2 y.o., running up and down the carriage playing with local kids. Our carriage was full of vietnamese families who loved our curly blond little one and the time passed quickly. There was a dining car at the front where you can sit and some ladies came through with a trolley of snacks and drinks. There were power points by most seats so you can charge devices if you like. Shelving above the seats fits most luggage but we were able to store our larger cases on a shelf as you first walk into the carriage. Overall the trip was easy and there were heaps of cabs at phan thiet station to whisk you to your accomodation.
For our trip back to saigon airport we hired a car through our accomodation. The cost was $110 and for that we got a big new 4wd and an excellent driver. The trip to the airport on a Thursday morning took @4.5 hours. I actually preferred the scenery from the car than the train and we had a short stop halfway for toilet and food break. Unlike other posts we didn't find the trip by car to be uncomfortable or 'scary'. I thought the roads were good overall and never felt unsafe when overtaking but l do admit that it can make you want to close your eyes sometimes!
So obviously your choice of transport depends on number of factors but l can recommend both forms without hesitation.