Definitely the 21 pleats. It's not so much the pleat depth - it's the weight of the tartan in the back that drives the swing. The kilt will look better with pleats that are wider at the hips, and you'll get better swing because you'll have more tartan in the back of the kilt.
And 21 pleats is fine. I just finished a kilt for someone with 42" hips, and I only put 20 pleats in the kilt. The tartan has a very large sett, and there were no opportunities to either pleat to a stripe that occurred more than once per sett or pick up more than one pleat per sett in pleating to the sett. The kilt looks perfectly normal.
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