Should I go for Scathach?

Yes if she's your waifu, and for current content she's arguably the best. But, Karna and Bryn are more future proof imo, because Buster and Merlin.
Karna will have Buster resist down (AoE) on his NP after Rank Up Quest which basically increases the damage dealt by your servant buster cards (any servant on that party), which is nice.
As for Bryn, from her recent Interlude on JP server, her 3rd skill becomes broken and everyone literally can become Pseduo JAlter with her skill alone.

If yours SQ are limited, go for something you need more; you already have an AoE buster lancer and a ST buster Lancer.