About Giratina vs Lugia
I will be comparing the two in terms of how well they do what they can feasibly do. After all, you don't expect a wall to sweep much do you?
Both Lugia and Giratina take about the same damage from special attacks with the standard "(The) Great Wall" sets listed in the Smogon analysis, assuming the attack has the same effectiveness on both of them. However, of the most common (special) attacking types in Ubers: Ice, Electric, Dragon, Dark, and Water, admittedly, four are super-effective against Lugia, while only three are super-effective against Giratina, and it resists the other two. Therefore, in this respect, I agree that Giratina is superior, but I suppose if you factored in Roost vs RestTalk...
Next, in terms of physical walling, which is what you should really be doing with these two. Giratina weathers neutral hits better, but nearly all physical attackers (Groudon, Rayquaza, Garchomp, and various other threats) can hit it super-effectively, often with STAB moves. Giratina is helpless against Rayquaza and Garchomp, but it takes on Groudon fairly well. However, a mere 31% of Groudon elect to run Rock Polish, meaning that Giratina has fairly limited uses. Even then, if Giratina's Will-o-wisp misses, Groudon can most likely take out Giratina. Both Lugia and Giratina can weather hits from minor physical attackers such as CB-Scizor, the occasional Metagross, and random unboosted (physical) moves in general. Lugia on the other hand, can stop Garchomp and Rayquaza quite reliably, but generally falls to Rock Polish Groudon, although it has a chance if it is at full health. The most common physical attacking types are Dragon, Ground, and Rock, so Lugia is weak to one, neutral to one, and immune to one. Giratina is weak to one, and neutral to the others. Outrage however, is significantly more powerful than Stone Edge, and often carries STAB and has 100 accuracy, so that must be considered as well. 
Now, where Lugia really outshines Giratina, the movepools.
First off, Lugia has the ever-coveted reliable recovery move in the form of Recover/Roost, which clearly outclasses RestTalk. Secondly, it has access to both Reflect and Light Screen, while Giratina must rely on a 75 accuracy move for physical attacks and has nothing for special attacks. Both can Phaze thanks to Roar/Whirlwind, but Lugia gets one more move due to Roost taking one slot and RestTalk taking up two, which lets Lugia either Toxic, Thunder Wave, Light Screen, or maybe even Calm Mind. Lugia's movepool is undoubtedly superior to Giratina's.
Both can run a Cro set (Calm Mind, Rest, Sleep Talk, Dragon Pulse) set reasonably well, although Lugia gets Calm Mind, Roost, Whirlwind, Ice Beam, so it is arguably better, due to freedom of moves and no sleep to set up on.
*About Giratina threatening to Outrage Wobbuffet, a defensive Giratina is highly unlikely to run Outrage due to its low base Attack and Speed hindering its sweeping potential, and the side-effect of Confusion. Wobbuffet can encore, take a hit, and switch to something that resists Outrage, or can set up on it, so it really ends up the same way as Lugia does.
After a whole bunch of Theorymon, I hope I have discussed the two in an unbiased (probably not) way. I leave the decision up to the reader as to which wall is better. In general, Giratina has specific uses (Blocks Rapid Spin, takes on RP Groudon) and Lugia has more general uses.