Hoshidan Xander

Do you plan to eventually go for a +10? because if not I fail to see why you are so set on merging a 5* locked seasonal dancer. That said I wouldn't worry about speed as Armors can double anyway thanks to those BS fighter skills. Dancers are more support units, so unless you plan to give him Ryoma's Dagger he probably shouldn't tangle too much with Armor units and their high defenses anyway.

You dump his speed, no question about it. With 16 res he should not be going up against any magic damage.

And isn't he a seasonal? It's your choice, but I think those other two are fodders. His BST is appallingly low, especially with the superbanes on Res, Spd, AND HP. What a dick move by IS to give him a superbane on 3 stats, and they are the ones you would consider dumping. The -4 to any of those stats puts him in the bottom BST bin on top of that.

That's an easy -spd, because it's a bane that affects his performance the least. Xander's thing is physical tanking on enemy phase and he shouldn't be taking a single magic hit, due to his horrible res. The argument about winter Tharja doubling him makes no sense in this regard (because he should be staying as far away from any magic) and taking -def because of that is also not a good thing to do, because you'd be making his only good matchup much worse (as he would take 6 more dmg in every fight, which can mean a difference between life and death in his case).

-spd puts him at a worse arena scoring tier, but even if he stayed in the 140 bin, he's not a high scoring arena material, so arena really doesn't matter here.

Definitely go with the -Spd one. You are right, with that speed he gets doubled by absolutely everything. But that doesn't matter. The only units around that speed tier are Winter Tharja, Arden, and Reinhardt, I cant think of any more, but neither of these three even matter, since you arent getting Xander anywhere near these units - Reinhardt could kill him regardless of him doing a Quad attack, Tharja, I don't know, but as an Armor I find it unlikely you will have Xander as a pure physical unit deal with her in any case, and Arden has his Follow-up Ring so he would double you anyways.

In short, any unit that gains an additional attack from using a -Spd Xander would be the doom of him anyways. And against regularly paced units, who are sure to double him anyways, you definitely want every point in defense you can to survive that 2nd hit. He's on 40 Def with the Wave, 41 if you merge him, meaning he can survive an enemy of up to 60 Attack. With -Def, this goes down to 54, which is a range a good deal of units still reach. I'm sure you don't want that.

His speed is very low. Like very low. He will never double or avoid being doubled. So minus spd. Not even a choice.