depends is this for competitive play? Anyway I personally think that its your choice if you want to be a tank,sweeper,or just an annoying Pokemon in all but if you were a tank I would have it preferably in defense form. If you want to be a sweeper then have him in attack form with somthin like phycho boost and maybe one or both of those and I would go with assault vest so he can tank a little and if you want to be annoying go defense form with phycho boost recover spikes and somthin else.
P.S I'm basically saying depends on what you want deoxys to be and what ever and I'd go with the move that counters the poke so if you wan a be a special tank go with mirror coat if you want physical tank the go counter
hope this helps :D:D:D:D