In my opinion, both ELISA kit seems suitable for total brain homogenates. Personally, I prefer Millipore as a more "trustable" company. If your are interested to visualize where the IKappaBalpha mRNA expression is up or down regulated on mouse brain sections, I suggest you to use in situ hybridizations. In our lab, we systematically use this technique when measuring the inflammatory response. See the reference bellow.

Laflamme, N., Lacroix, S. & Rivest, S. An essential role of interleukin-1beta in mediating NF-kappaB activity and COX-2 transcription in cells of the blood-brain barrier in response to a systemic and localized inflammation but not during endotoxemia. Journal of Neuroscience 19, 10923–10930 (1999).