As I recall the move is successful in both cases. What happens when you attempt to perform arith on the field depends on its contents. Also remember when you MOVE a numeric field to another numeric field of a different type (e.g. COMP or COMP-3) arith processing takes place. That is, the data is converted, as it is in arith functions, and that's where the error will occur.
As you've already noticed, you got two widelly different ans to your question. It's a ques that's easily answered by creating a small pgm to test both scenerios.
You should try to take that approach wherever you can. The computer is the final arbiter in all these situations. If the computer says it's so, you can take it to the bank.