Oh lol....an eccentric ditto. Of course....all stats are copied after the transformation, barring HP. It also copies Nature/IVs/EVs....blah blah blah....so basically, Choice Scarf FTW on Eccentric ditto. However, if you are going to use ditto in a sand team, There is a bit of a disadvantage if you switch in to a non rock/steel/ ground or at least Excadrill(Dory in Jap......) and revenge kill it.....By the way, if you switch in to Dory inside Sandstorm, watch out for EQ....and if you don't mind, if you kill it, you'll have a super fast Dory, with a speed of:
Jolly Excadrill 252 Speed max IV and EV: 302 X 2(factoring sand) X1.5(factoring Scarf):
a pretty fast 906 speed............
however; they might predict your move and send a flying type in.......meh.
Hydreigon.....with Choice Scarf......hmmm.....it just depends how your versatility would be. This guy could end things fast with a choice scarf, and with his massive sweeping stats, with acceptable speed (although sped by most OU), equipped with Scarf, this thing could win.
Now, for picking......and I almost forgot! I am not sure if Ditto still has the same PP mechanics as the last Gen, like having only 5 PP on all moves. So:
Usefulness in terms of offensive: I think Ditto should only be sent out to copy a pokemon if that pokemon is a sweeping one; if you send him out to a staller, CS Ditto would be useless. Hydriegon, on the other hand, can come in to anything, baring dragon, bug, fighting and ice. They can both function as revenge killers. The Surpirse factor of Ditto is a bit limited; if you end up not using CS Ditto, a speed tie happens, and if you end up going first, you can play mind games with your opponent.
In the end, the comparison of both can be a bit hard to do; it will just matter about the usefulness of which pokemon on your team, either a counter, or a supporter.