Aoe Saber

Unless pairing Seig with George go with Gawain the buster gorilla.

Gawain, out of these two (unless dealing with dragons). If I'm not mistaken, the only lower rarity AOE Saber for now is Fergus, though I don't know whether he's good or not.

Siegfried is a dragon slayer and most dragons are Rider class so his skillset is pretty useless while Gawain is a better option since his damage will increase with sunlight but both aren't really a good choice since you cannot bring out their full potential. If you have the option just don't use them but if you like them then by all means.

For future singularities, Siegfried will be far better than Gawain due to NP strengthening. Since Gawain has a BBB deck, it is hard for him to generate NP unless you put Kaleidoscope on him, whereas Siegfried can potentially use NP back to back with arts crits and NP gain boost from his skill and NP.

That said, I deploy my NP2 Siegfried in every lancer node with Lv100 Golden Sumo equipped, dealing 50K-60K damage per enemy on NP. When fighting a lancer dragon, with 400% OC he can reach 250K-300K or more in a single unit and that is pre-Merlin damage.

For AoE wave clearing, I generally borrow a support Altria (Alter), a 4*. She hits like a truck.