Auto-crit damage will FAR outstrip the sneak attack damage by 20th level. Not even close. Let's talk about some unoptimized numbers for a minute.
Rogue 20 with Str 22 has a +6 modifier. He's got Power Attack, Leap Attack, and Exotic Weapon Proficiency (Mercurial Greatsword). When he attacks with his +5 Mercurial Greatsword he Power Attacks for, let's say 5, and leap attacks. His damage on a successful hit is 2d6+34, and with the auto-crit that turns into 8d6+136. That's one attack and it already exceeds making five sneak attacks or +50d6. And the numbers only go up from here.
Wait, what? How is that not optimized? Non-optimizers wouldn't have even heard of Leap Attack and Mercurial weapons...
Mercurial Weapons maybe not, but Leap Attack is hardly obscure. It's right there in the first supplement any non-optimizer was likely to look at. Instead of the Mercurial Weapon, the Rogue just uses Scythe then, as has already been mentioned. The difference in damage is absolutely negligible and it still outstrips a full sneak attack routine in one hit. It's not optimized* because the damage can keep on ramping up by A LOT.
Not that I think this is a bad thing, I'm just arguing against SneeR who is saying that the auto-crit is a nerf to the damage, which is only true if we're talking about Rogues running around with Str 10 and attacking with (non-punching) Daggers.
*EDIT: Of course, if you want to compare the +50d6 sneak attack with full BAB + haste numbers to I'm a complete newb auto-crit numbers, then let's. Some weird build somehow has full BAB +10d6 sneak attack and all day haste. let's say that costs him roughly three feats and some class features. If he hits with all five of his attacks, we'll say he's got Str 28 and is using a +5 Greatsword for great justice, then he deals 60d6+18 damage, or an average of 228 damage, right? Okay, so now we have bum fuck Rogue 20 with auto-crit
TM, with Str 22 and Dex 24, who took for his feats, Weapon Proficiency (Scythe), Weapon Focus (Scythe), Iron Will, Exotic Weapon Proficiency (Hand Crossbow), Exotic Weapon Proficiency (Repeating Crossbow), Endurance, and Diehard. Whatever. If he hits with one +5 Scythe attack he can use one of his 10 auto-crits
TM per encounter to deal 8d4+56 damage, or an average of 76. Of course, that's just one attack, so if he hits with three attacks that's, what do you know, 228 damage. So, in conclusion, we have a fictional, non-existant, but still optimized build dealing the exact same damage as a remedial D&D Rogue with auto-crit
TM.