Tough to say whether one would plane earlier than the other ... comes down to how much work you put in to it.
I have both the 127 / 117w. Raced the 117w on the PWA in 2012, and the 127 in 2011. My consensus is that I can get both boards on to the plane in the same low winds; I may just have to work a little more with my legs on the 117w to make it really pump up on to the plane... once planing in almost all wind conditions I find the top speed to be almost identical on both boards...
Where the BIG difference lies in is planing out of gybes and staying on the plane in lulls. The extra width on the 127 makes a big difference here and if you do a gybe in very light winds it's easier to keep the momentum and keep on the plane where the 117w can sometimes feel a little small and you can feel the fin depower under your feet when it's really, really light. (ie, sometimes on a run i may have to take my back foot out of the footstrap and place it in front of the strap on the 117w to keep it on the plane if it drops below 10 knots, whereas on the 127 I could just keep that tiny bit of fin pressure to keep my foot in the strap).
I will continue to race on the 117w instead of the 127 though as the tiny bit of extra lightwind ability of the 127 is negated by the 117w being A LOT easier to gybe when it's powered up as I'm not such a big rider.
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