Undo button, GDI and Nod research order

The first part is actually a minor suggestion that allows a player to refund his research and credits from any unit he has researched. Its a 75% RP and 80% credit refund. It may be good in case of accidents where something has been researched when it was not required.

Now unto the second one... What would your recommended GDI and Nod research orders be? As GDI after Pit-bull, do we get straight to fire-hawk or take a detour on the predator? As Nod, do we do the same thing with black hand and vertigo?

One more question, of the 4 research-able anti-vehicle units early in the game, are some of them worthwhile compared to the others? I'm talking about the offensive Predator, Black Hand, Zone Trooper and Scorpion...