I have a friend who owns both, and she and I have always felt the 23 has a warmer sound while the 30s we have heard sound brighter. Of course, that kind of thing is very subjective.
There is also a slight difference in the quality of the wood used. The more expensive the harp, the longer it is aged. Some 30s have a big sound, suitable for orchestra. My friend says to her they are all tight. The ones I have known have all been very different from each other. I think 23s are more consistent, have a well-rounded, warm, pear-shaped tone.