Is the main nose gun on the variants the same or different than the Warden? People keep speculating the Harbinger has railguns, but I see no text that confirms that.

Yes! All three Vanguards will have different nose guns. The weapons for the Harbinger and the Sentinel are still in development with the component pipeline, so they may different somewhat in appearance from the versions used in the concept art. The central gun on the Warden is an APOC Death Ballistic Gatling gun, while the central guns on the Sentinel and Harbinger are a plasma gun and a mass driver rail gun, respectively. (Manufacturers have yet to be assigned.) The four secondary guns are also different. The standard model uses Behring M4A lasers, the Sentinel has two distortion cannons and two MaxOx electron guns and the Harbinger has four higher damage lasers.