Blue mage

I think Delthea is better she has the highest Atk on player phase

Would say from first look Delthea. Should you run Infantry heroes with Brave weapons 100% Delthea her tome synergy is to amazing to pass.

Your Mae is really good though. She can run a proper blade build.

Your Lucina / Caeda are bad IVed.
Robin is not that interesting with neutral IVs, even though he's not really bad.

Delthea will also be a strong character, but not as a blade tomb :)

I also agree with picking Delthea. She will be always useful thanks to her tome.

Use both. Mae for blade tomes and Delthea for nuke/support. I recommend ditching S!Lucina's Swift Sparrow on either (your choice).

Your Delthea and Mae are set-up well for fast nukes. Others have covered their strength well already so I'll just add that Robin fills a different role. He's a physically defensive mage that out of the box gets weapon advantage aganist reds and colorless. I run Triangle Adept on mine and it makes him an excellent counter to archers, as well as reds with DC like Ike and Xander (doesn't fear their counter).