Well if the difference is between 25 commerce and some Worker turns (how many?) the 25 commerce is likely better, *unless* in ~10–15 turns you have a sudden influx of new resource tiles and not enough Workers to improve them all.
What's the reason for not improving both? Could the extra food let you whip a Settler sooner?
What's the reason for not improving both? Could the extra food let you whip a Settler sooner?