1/2" drive is the way to go, the only advantage to the 3/8" is it's going to be smaller and lighter. With the major disadvantage of much less torque.
are there any adapters like in regular hand ratchets to downgrade from 1/2" to 3/8"?
1/2" to 3/8" adapters are readily available on all the tool trucks, and most other places that sell tools.
Here's a few examples, not only are the readily available they make them for impact wrenches.
Snap on Adaptor, 3/8" Internal drive x 1/2" External drive, 1 5/16"
Bottom right of this page in the Snap on Catolog
Again this one is made to be used with an impact wrench.
The ones pictured below are NOT for impact wrenches
Craftsman 4 pc. Adapter Set
While this one is for use with impact wrenches.
Craftsman 1/2 in. to 3/8 in. Impact Socket Adapter
It's is important that you use impact sockets as non-impact sockets can shatter when used on an impact wrench.