I decided to go with the 3 groups of 2 tubes for the front of the clock, since it will be easier to separate the hours, minutes, and seconds this way. I might change back to all 6 in a line in order to make the case easier to produce, though. I'm using IN-12B nixie tubes, so there is a little dot on each tube that I can access to make the hours, minutes, and seconds easier to tell apart.