The Atom is a watch while the VT3 is a puck (as in hockey puck to approximate its size). The Atom can be worn as a watch and it's designed that way. The VT3 shuts itself down just like all other non-watch-type dive computers.
It's called Aeris and the Atom's equivalence is called the Epic. The VT3's equivalence is called the Elite T3.
Though these computers are exactly the same, sometimes Aeris dive comps run about $50 cheaper than Oceanic. Don't know why, but that's the way it is sometimes. I went with the Elite T3 over the VT3 because it was $50 cheaper for the exact same thing.
And yes, you can buy the computer first and then a transmitter later if budget is an issue.