for what its worth, the gengetsu is not well suited to the use of honing rods... a finishing stone or a strop would be far better. If you insist on using a rod, a smooth glass or smooth ceramic rod would be the best (but i still dont like them). Use one with exceedingly light pressure.
i would say either get a 50/50 one, a real left handed one, or choose a 150mm petty that is on the tough/durable side of things. The petty is probably most versatile, but you will need to be a bit more careful.