From my car stereo days I learned that the lower the ohms of the sub the deeper and harder it hit. You could have the same two sub-woofers, one wired for 2ohm, one wired for 4ohm, the 2ohm one is definitely hitting harder and reaching lower tones with the same amp.
I would think the same principals apply to a guitar speaker in a cabinet. I think since you can use so many different ohm settings, like on my JSX for example I can switch between 8 and 16 (and I think even 4 if I remember correctly), and then my cab has 8 and 16, the best thing to do is to try them out and see which one suits the amp and the playing that you are doing more.
Using a lower ohm setup will be a thicker sounding, bassier type of sound. The higher the ohms, the "thinner" it will sound but be really clear. I would expect to use an 8 ohm setting for something like a Recto, but then a 16 would work or an 8 would work for a 5150. I know from my days at the studio I had everything hooked up for 8ohm in our cab room and our rack of heads. Some heads we messed with using 16ohm and they were coming out better for aggressive playing, some weren't.
Try it out and see...that's a common line around here for stuff along those lines.
~e.a
