Aegislash also has stance change as another pro, so it can have amazing offensive stats and defensive stats at different times, and while girantina has respectable attack, it's mostly just a wall. I dint know how much that matters, but I thought I should bring it up
Edit giras offensive stats are good, but aegis are higher
150/100/100 isn't that far off from 50/150/150 tbh
252 Atk Tyranitar Crunch vs. 252 HP / 252+ Def Giratina-O: 194-230 (38.4 - 45.6%) -- guaranteed 3HKO after sandstorm damage
252 Atk Tyranitar Crunch vs. 252 HP / 252+ Def Aegislash-Shield: 146-174 (45 - 53.7%) -- 1.2% chance to 2HKO after Leftovers recovery
252 Atk Tyranitar Crunch vs. 252 HP / 0 Def Giratina-O: 270-318 (53.5 - 63%) -- guaranteed 2HKO after sandstorm damage
252 Atk Tyranitar Crunch vs. 252 HP / 0 Def Aegislash-Shield: 188-224 (58 - 69.1%) -- guaranteed 2HKO after Leftovers recovery
In fact, it would seem that Giratina has more bulk on both ends of the equation. Let's see how they pan out on the offensive end of things:
252+ SpA Griseous Orb Giratina-O Shadow Ball vs. 0 HP / 0 SpD Tyranitar in Sand: 54-64 (15.8 - 18.7%) -- possible 6HKO
252+ SpA Aegislash-Blade Shadow Ball vs. 0 HP / 0 SpD Tyranitar in Sand: 54-63 (15.8 - 18.4%) -- possible 6HKO
252+ SpA Spooky Plate Aegislash-Blade Shadow Ball vs. 0 HP / 0 SpD Tyranitar in Sand: 63-75 (18.4 - 21.9%) -- possible 5HKO
So if aegis wants to have better offensive stats then giratina-o, he needs a boosting item, which means forgoing lefties.
Given that the difference in the two's bulk is relatively similar to the differences between the two's offensive stats, I'd consider them equal from a pure BST point of view(barring speed, I demonstrated the importance of base 90 vs base 50 in my previous post, it's significant).
It's also worth noting that due to base 50 hp and base 50 speed, aegis basically
needed HP investment (barring scarf lure sets). Giratina-O, on the other hand, can actually do without hp investment due to having 441 hp uninvested (more then 252 hp mew!). This allows him to either invest in defenses, speed, and/or offenses as the user pleases, basically making him the OU equivalent of Arceus in terms of being able to pull off pretty much any set he darn well pleases.
Incidentally, this also separates him from hydreigon, who lacked the bulk to be able to completely forgo hp investment and still expect to take hits (because he could never take hits in the first place...)