The Rebbe Maharash (Toras Shmuel 5632 vol. 2 pg. 377) notes this discrepancy between
the two terms used for "rock", and explains it based on a Midrash (Yalkut Shimoni Chukas Remez 763):
והכיתם לא נאמר, א"ל כשהנער קטן רבו מכהו ומלמדו כיון שהגדיל בדבור הוא מיסרו, כך אמר הקב"ה למשה כשהיה סלע זה קטן הכית אותו שנאמר והכית בצור אבל עכשיו ודברתם אל הסלע שנה עליו פרק אחד והוא מוציא מים מן הסלע
"Speak to the rock, do not strike it. G-d told Moses, 'when a child is young, the educator may [at times] hit the lad in order teach it. When the child grows into adulthood, however, the educator must rebuke him only verbally. Similarly, when the rock was but a 'small child,' I instructed you to strike it; but now [after 40 years when it has grown larger] you must only speak to it. Teach it a chapter of Torah and it will produce water."
He continues to explain how these two concepts correspond to two levels of Jewish souls. The incident in Parshas Beshalach occurred shortly after the leaving Mitzrayim, when the Jewish nation were still considered "young" and likened to a צור. However, the story in Parshas Chukas was after 40 years of the Jews being in the desert, and had by then reached the level of a סלע. Thus, Moshe erred by treating them in a manner that was appropriate for a צור.
See the original discourse at length, or here adapted in English by Rabbi YY Jacobson (thanks @Menachem).