It's based upon how the speakers are wired up (series vs. parallel). However, if the cab says 8Ohms like this one:
http://www.guitarcenter.com/Marshall...16-i2034726.gc

then I think you should just connect it to the 1x8 as it states it is an 8Ohm cabinet which I read as the overall impedance.

I have a Mesa 2x12 that has two 16Ohm speakers, but they are wired in parallel so the effective load is 8 Ohms so I connect it to the 8 Ohm output jack.