Mewtwo is harder solely for the fact he is level 70 while Deoxys is only level 30. Mewtwo also has safeguard and recover, which can thwart your catching strategy easily.
If you want to judge for yourself:
Deoxys Lv. 30 Mewtwo level 70
Knock off (DRK) Swift (NRM)
Spikes (GRD) Psychic (PSY)
Psychic (PSY) Safeguard (NRM)
Snatch (DRK) Recover (NRM)
putang ina tlga
Palkia, Lugia, Dialga, Deoxys, Mewtwo, and Mew
First you have to catch Mewtwo and then head to Mt Moon at midnight on Monday. Somewhere deep in the cave is were the deoxys are.
Articuno, Zapdos, Moltres, Mewtwo, Mew. By trade you can catch all Regis, Deoxys, Latios, Latias, etc.
First you got to catch him in Pokemon leafgreen or fire red and trade. Or you can use a gameshark. it lets you catch all the Pokemon from bulbasaur to deoxys.
To get Deoxys, you need the Aurora Ticket from a Nintendo Event. To get Latias, they are in the wild after you beat the Elite Four. To get Mewtwo, you need to trade from LeafGreen or FireRed.
You get Tornadus from the Dream Radar app on the 3DS, or transfer it over from another game.
You must catch every Pokemon 1-493 except for Mew, Lugia, Ho-oh, mewtwo, and Deoxys.
Deoxys...
You catch deoxys at birth island.
You can't catch Deoxys in Silver..
You can't catch Deoxys in soul silver