I think you pretty much answered your own question--which is more important to you--image quality or traveling light? The difference probably isn't as big as you're thinking, since the NNs break down into pretty small packages, all told. I tend to keep my NN3 disassembled with the horizontal and vertical arm assemblies separate, so you're packing two bars, vs. an L-plate.
I would also state that I use a Nodal Ninja 3 with either my 5DMkII+Sigma 8/3.5 circular fisheye or my micro four-thirds GX-7 + Rokinon 7.5mm f/3.5 fisheye, and have no trouble with either, so getting a full-frame diagonal fisheye (say, a 15/2.8) might be an alternative to using the 12-24 and still sticking with the smaller head, but still having higher quality than the 7.5 will get you on APS-C (although, gotta say. It's no optical slouch--better performer, overall, for me than my Sigma 8/3.5 when used on APS-c).
But. With your NEX+mft Samyang/Rokinon 7.5/3.5 fisheye + adapter (if the fisheye lens's hood is shaved) is actually a popular combination among equirectangular shooters, because, like my 5DII+Sigma 8mm, you get 180° HFoV, and it can be handheld and only requires 4 shots around at 90-degree intervals to cover the entire sphere. A simple plumbline might be all that you need with that combo.
Obviously, however, if you swap to an ultrawide rectilinear lens, you're going to need more precise rotation and a lot more shots to cover the scene, and a panohead and tripod are more likely. And you're liable to be able to use the NN4 with both setups.
