I would go for the 1/40 scale Imai models. The Imai kits have white metal joints,better movability, missiles and are better transformable. Its also closer to the animation design. Therefor the Arii kits have a nice looking cockpit and the chest armor look better. The legs are square-edged. The plastics used for injection is very brittle and the joint covers and housings are very thin. They could break easily. Details, decals and panel lines on both kits are nearly the same.
The Imai 1/40 Orguss Kei Special is also available from Revell. They named it "Nebo" and sold it as a 1/48 Robotech kit.