DD jolly tyrantrum at +1 with life orb and without:
+1 252 Atk Life Orb Tyrantrum Head Smash vs. 0 HP / 0 Def Mew: 452-534 (132.5 - 156.5%) -- guaranteed OHKO
+1 252 Atk Tyrantrum Head Smash vs. 0 HP / 0 Def Mew: 348-411 (102 - 120.5%) -- guaranteed OHKO
RP adamant tyrantrum with life orb and without:
252+ Atk Life Orb Tyrantrum Head Smash vs. 0 HP / 0 Def Mew: 333-394 (97.6 - 115.5%) -- 87.5% chance to OHKO
252+ Atk Tyrantrum Head Smash vs. 0 HP / 0 Def Mew: 256-303 (75 - 88.8%) -- guaranteed 2HKO
jolly tyrantrum at +1 hits 397 speed, adamant tyrantrum +2 hits 482 speed.
jolly without life orb at +1 out damages adamant with life orb (big surprise but I wanted to do the calcs anyway) meaning that you'll have much fewer mons that can switch in to you. at 397 speed you out sped 130's with a + nature (again, already been said but I just wanted to calc it for myself). so basically you only get out sped by 6 mega pokemon (lopunny, manectric, sceptile, alakazam, beedrill and aerodactyl) and choice scarf users (I won't claim to know every choice scarf user or their abundance in the meta) after a DD. this means that DD tyrantrum can get away with not running life orb to run lefties or a berry making it less vulnerable to revenge killing, while still having a significant punch and only having to worry about a handful of mons being faster. since it's unlikely that your opponent will be running more than one (two at the most? I've never seen three or more) scarfers it's just a matter of eliminating them with good partners. I would also like to state that while rare, situations may arise where a second dance is possible, but a second polish is essentially useless.
Because of all this I believe that DD is better against most teams than RP.