Zephiel or Xander

I would say Zephiel, simply for the fact that he has (neutral) 24 Res compared to Xanders abysmal (neutral) 17, wich is the second lowest value of all sword users.
On the other hand you get DC for free on Xander while you have to feed Zeph a Hector if you want it.

If you give Zephiel distant counter, he will function much better as a defensive green mage check compared to Xander since wary fighter prevents him from getting ORKO'd by the blade tome mages, something which Xander has difficulty reliably performing due to his low res/spd. However, Xander more versatile as he's able to engage on the green mages due to his superior movement, and the access to cavalry buffs make him an excellent pick if you can run horse emblem

of course it depends on your team. cavalry? then Xander no doubt if not then Zephiel might be the better decision (especially with Distant Counter). In term of versatility Zephiel is limited because his support skill has to be Pivot most likely and his B passive has to be Wary Fighter... Xander can be build how you want him to act. From my experiences I can tell Xander (with QR and Swap inherited) saved my ass in this arena season a lot more often than Zephiel did in the last - but that might be a reason of my play style...I don't really like those slow units very much because you need at least one unit (dancer, someone with reposition skill or one to use Pivot on) to make them move fast. Most of the time you can't use this unit to help the others and I prefer them to help on the frontline. In addition AI likes to change positions very often in enemy phase so I sometimes struggle with armored units to react because of their low movement range. It ends up that I try to kill all the enemies I can with the other three and let the rest run into that armored unit.
If you look on the defensive side Zephiel is the king. NO NE neutral vanilla unit can OHKO him. Xander is OHKO by every blue magic damage dealer (except Nowi) plus Abel, Azura, Cordelia, Catria and even red mages like Tharja, Tiki (Y) and Celica can kill him easily. so while Zephiel doesn't care if you manage to take him out of the danger zone (if he only faces one threat) Xander does.
If you like the offensive play style then Xander might be your choice because you can better keep your units together and Xander is great in getting rid of those annoying Nino's and Julias which Zephiel barely can because of his movement.
So long story short: if your team struggle with fast (moving) green units (Nino, Julia, Cecilia, Spring Camilla etc.) than Xander is your go to guy. If you have another red attacking unit or the rest of your team can handle those threats than Zephiel is better. If you have Distant Counter for inherit I would also go with Zephiel.

Zephiel is definitely the more versatile option. While Xander has Distant Counter built into his weapon, he's only better overall than Zephiel when he's on a Horse Emblem team. Zephiel with DC obliterates any Green Mages that come his way. Personally, I'd use this set:
Zephiel
Eckesachs
Pivot/Swap/Reposition
Bonfire/Ignis
Distant Counter
Wary Fighter
Threaten Def 3 (or something else)

It doesn't cost too much SP or heroes to get the skills on him. Also, if you're sacking a Hector, make sure you give Zephiel Goad Armor in case you want to build an Armor Emblem team (I think you need to have Spur Atk 1 on Zephiel beforehand, so be wary).