You can use either on TRAXXAS engines, according to TRAXXAS.
Personally, I use oil labeled as After Run Oil for storage, and I use WD-40 when cleaning my parts.
Which is better? I'd say the ARO is best for storage as it tends to coat well and last longer. WD-40 will coat, but tends to "evaporate" over long periods of time. Also, there is some debate as to whether or not WD-40 is friendly to the o-rings and plastics in the carb.