I would say that your best strategy is to buy the IXB as soon as possible, the larger torpedoe load compared to the VII types will allow you to amass prestige points a whole lot quicker, regardless of what you plan to buy with them later on.
So: buy the IXB, save enough points to buy the IXC and spend any excess points that you earn while waiting for the IXC to become available on the crew (getting better crewmen) so that when the IXC becomes available you'll be good and ready for it, transfer your top notch crew to it
