In the playoffs (~110 minutes so also a small sample size), the Celtics were outscored by 2 points when Romeo was on the floor, compared to being outscore by 54 in the ~130 minutes he was off the court. This compares to the Celtics being outscored by 8 points in the 75 minutes Nesmith played and 48 points in the 165 minutes he didn't.
I don't think there is any conclusive stat that is going to settle the argument as to whether Langford or Nesmith is the more valuable, higher ceiling prospect or who is the better player right now. Neither has done enough for long enough for it to be considered a track record.
But these plus minus stats, regular season vs. playoffs, are indicative of why it is hard to peg exactly what Langford could be. Langford played his best basketball in the playoffs in 2019-20 (based on my subjective interpretation of what I saw on the court) and also played decent in the playoffs against the Nets. He has it in him. You see it at times, just not all the time. He has been very much two steps forward, one step back (or three).
Nesmith to me has been more of a somewhat steady improvement without the periods of regression. Coming out of college, I understood he was a natural shooter but maybe not a high level athlete. On the Celtics, he has been the opposite. He flies around the court and has surprising (at least to me) hops but has not developed a consistent shot or a quick enough release.
If I had to pick one to be included in a trade right now, I would send out Langford. If I had to guess who will play more meaningful minutes at the start of the season, that might be Langford (maybe not the case by the end of the season though).