Which W! Tharja to keep?

i would go with +Hp -Spd, you dont need speed at all and having +Hp would increase both her physical and special bulk

I would think you should keep +res since it would make her magical bulk even stronger and make her glacies/iceberg even bigger nukes

+res -hp
Even without a spd bane, she will always be doubled by other units. Mathematically speaking, +res would be more effective than +hp, since it will reduce magic damage taken two times more than +hp

Both can work fine, but if you use +Res -Hp Tharja for bait more specifically Magic Atk, The Res Boon + Distant Defense 2/3 Seal + RES weapon upgrade + Glacies= you can kill some blue units(The Robin in TT for example), and if is not a blue unit some times you dont loose HP.