Normally I would lean towards just buying what you need. Why spend extra fuel to tow around extra weight? However, in this case, you are only talking about 2 extra feet of length. You will certainly need the extra space some day and perhaps even save an extra trip because you got everything on the same load.
For me, longer trailers are easier to manuever then shorter trailers. Keep in mind that if the axles, springs and tires are the same for each trailer, then you will have less cargo capacity with the longer trailer. Should be a minimal reduction though.....