I had a good discussion with an Anon about Rena's identity last time, but couldn't reply to it, so here's the continuation.
When people thought Fake Itsuki is Ichika, the Rena theory got "confirmed". Now that she is revealed to be Miku, "Miku = Rena confirmed." It still can go either way, so one of them "making more sense" or being "logical" should not play a role here. Heck, it could easily be someone other than them, and Negi still makes an explanation anyways even if it's bullshit to us.
Ichika got her main conflict dealt with this arc. So why her? Momentum breaker. I would say Miku got the momentum after the Field Trip. Then Nino comes in and sweeps, before Ichika could have made her move. Now everything points towards Ichika getting some sort of upper hand next, since Miku is fine with being student and tutor, but changing it by the time. I find it more possible for the Rena reveal to happen with Ichika, if she starts being confident and aggressive like Nino, kisses Fuutarou and/or even invites him for a date. Something is gonna break her momentum again, and I don't think it will be a work related issue. Negi used that 3 times already, so I wonder what's left for Ichika conflict arcs? So my guess is Rena.
There's no guarantee that Miku's issue is disguise related at all. It's possible, but what she is good at is acting out her sisters. While Lolikano is technically one of them, Rena is still an act based on information. Miku pointing out to Nino their impersonations is not a bad clue, but since they did it to Fuutarou at least 3 times consciously, I also not consider it a strong lead.
Miku's conflict and potential career issues are still there. We have no idea at all about her dream. Her comment on Itsuki's makes me believe it's not history related, so it need to get tackled. So IMO, she doesn't need the Rena reveal to get future conflicts, while she can also deepen her relationship with Fuutarou.