I suppose I should clarify: I'd class even the likes of "Ripper Blast" as physical damage, since as far as I know it scales with Claude's P. Attack and his sword and all. Elemental physicals, sure, but hey, those are in FF8 too.
DL Claude deals ~.8 PCHP with Mirror Slice, so 3.2 PCHP with 2x Atlas Rings. He's equipped with Levantine which... is apparently about double the attack power of the Eternal Sphere (1600->3000). And huh, the stat topic gives Claude credit for Eternal Sphere? Not sure I would, it's perma-missable quite easily IIRC, so maybe I should hype DL Claude slightly less in the future. Anyway, it doesn't appear like it'd be a HUGE damage upswing (the damage loss for Sacred Tear [1250 pow] appears minor), but since it's multiplied by 4, it's pretty big, so we're talking 5 PCHP minimum, possibly more off Twink Claude's Mirror Slice. Anyway, Brothers does not have Vit junction, and IIRC Tweak League Quistis was at around 2.2 PCHP with the 100 Full-Lifes junctioned, so maybe more like 2.3 PCHP for Squall... but did FF8 cap HP at 9999? I'm pretty sure it did. If so, Squall is pretty well stuck there and the HP boosts don't matter. If it didn't, he's still getting into the "barely might survive Mirror Slice" territory before setting up Defend spam. If he does set up Defend spam... does Defend stay active when Squall's turn rolls around so that he can keep it up forever by just never entering a command? If it doesn't, then Claude might be able to Energy Sword spam stall into an opening.
Anyway, I'm definitely voting Squall here, but I don't think the supertank strat is gonna work, unless you see Brothers Squall as faster than Claude of course. (then lol Defend)