Mostly she is going to be melee oriented. On paper Cause Fear seems a lot better - affects one more enemy for the same slot, lasts multiple rounds, prevents them from closing for multiple rounds. On the other hand TMW is not concentration, is intelligence save and does a little bit of damage on a failed save. She will be concentrating on PEG, Blur or Haste a lot which makes TMW a lot more usable. On a thematic side note the character is Feylost so there could be a connection to Tasha.
Which of these would you get? I could also get both, dropping Cause Fear off my prepared list at 4th level (1 Death Cleric/3 Bladesinger)
Cause Fear is a much more powerful spell but as you say at the cost of concentration. It's a spell which can completely change how a combat is going, from deadly to easy which is why it's often worth that concentration. But if you have something else you want to be concentrating on, then Mind Whip is a pretty good spell particularly since as you say it upcasts well and targets intelligence.
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