Well, let's start out by talking about what kind of bulk they get from their stats.
He starts out with 106 HP. A solid and pretty average number for most ubers. Still really solid though. Paired with 130 defense and 154 special defense. Both of which are pretty darn high. Special defense being a little bit higher.
150 HP. This is super high, and HP matters when comparing bulk. 120 defense and special defense both falling below Lugia's defenses though.
Overall adding them together, Lugia is at 390 overall bulk, and Giratina is at 390.
So stats wise, they're overall equal, although Giratina is better off for a balanced bulky, or even considerably better for a physically bulky because of his monstrous HP.
Next is movepools for walling. This is where there is a large separation.
Dragon Tail, Light Screen, Recover, Toxic, Reflect, Rest Roost, Roar, Thunder Wave, Whirlwind.
These are pretty much Lugia's wall moves he gets. Dragon Tail can fight taunt while phazing and doing some damage, Light Screen and Reflect support your team. Roost and Recover for nice reliable healing. Roar and Whirlwind for 100% accurate phazing, Thunder Wave inflicts status. Rest is unreliable healing, but can be nice paired with a berry. Still not really recommended against Roost and Recover.
Dragon Tail, Hone Claws*, Magic Coat, Toxic, Pain Split, Roar, Thunder Wave, Will-O-Wisp, Rest.
Dragon Tail is phazing only on Giratina it can actually do some more decent damage due to it getting STAB. Hone Claws has the star because it's only really good because it supports Dragon Tail with accuracy and the power to now possibly KO some of the frailer dragons running around. Magic Coat deflects hazards. Thunder Wave and Will-O-Wisp inflict status. Pain Split is semi-unreliable healing, but isn't bad. It's better than nothing. Rest can be nice with a berry, but I still say it isn't amazing.
Okay, this is where you hit a separation. The main thing Lugia is holding over Giratina now is reliable recovery from Recover and Roost, and dual screens. While Giratina is getting things that are big damage dealers now. Will-O-Wisp is a crippler move for physical threats, and he gets things like Hone Claws + Dragon Tail making Giratina get some tanking skills as well. Being able to deflect hazards is also a nice little trick for Giratina in Ubers.
And lastly, we have typing. Which I'm not going to have to give you a paragraph about this to tell you the flaws. Lugia has nice typing for ubers, and has the special bulk to normally take some punishment from Kyogre fairly well, while Giratina gets screwed on typing. There are tons of dragon type sweepers in Ubers, and with BW2, we got Kyurem-B and Kyurem-W boasting 170 base in their special attack ( Kyurem-W ) or attack ( Kyurem-B ) stat. Along with Darkrai getting super effective STAB hits on him. Giratina does lose the battle of typing hands down. Though Lugia does get a slight disadvantage being weak to stealth rock, he's still immune to Toxic Spikes and Spikes, which Giratina has to deal with.
So in the grand scheme of things, I would have to say that Lugia is a bit better. Although they have a distinct separation you can use with them.
Lugia is better suited to be a special wall.
Giratina is better suited to be a physical wall.
With this in mind, I'll now give a set for both of them.
Keep in mind I'm making a special effort to give sets you wouldn't normally see, that are still effective. For standard sets just go to smogon or look at the moveset Q.
Special Wall (Lugia) @ Leftovers
EVs: 252 HP / 4 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Sunny Day
- Hidden Power [Fire]
This Lugia kills weather teams and steel types! Recover + Toxic will auto-wall-n-stall most any pokemon, but steel ( and poisons, but you don't really see those in ubers ) types can't be poisoned. Because of this, I introduce to you the common way to take out steel types, because they already have that 4X fire weakness. Adding Sunny Day makes your HP Fire pretty much guarantee a KO on these pesky steel types, while also giving the added bonus of taking out things that would hinder it such as rain. You can also screw weather teams.
Giratina @ Lum Berry
EVs: 252 HP / 252 Def / 4 SDef
Impish Nature (+Def, -SAtk)
- Dragon Tail
- Hone Claws
A nice physically bulky Giratina. Already boasting nice physical bulk, Will-O-Wisp cuts the attack stat of your opponent's even more. Dragon Tail + Hone Claws is a nice damaging and phazing combo so you can burn most of your opponent's team before going down, and Resto chesto is because Pain Split + Will-O-Wisp kinda screw with each other.