I know that the main concern with any instrument is the sound, but there are also practical considerations like, "Does this fit in my car?" and "Can I carry it?" and "How easily does it get out of tune?" Unless you're willing to buy a new car in addition to a harp, don't get anything bigger than 29 strings, because they don't fit unless you have a van, a stationwagon, or a Lincoln towncar. (In fact, when I got a car for my 16th birthday, we took the harp shopping with us to make sure it would fit.) The two Triplett harps I've owned, because of the soundboard construction, were really bottom heavy, and the case construction didn't help--they are hard to carry. Even though my Thormahlen 36-string is one of the biggest Celtic harps on the market, at about 26 pounds and with a good case, it's easy to haul around, besides being beautiful and having one of the most full and gentle sounds of any harp I've played.
Another question you want to ask is, gut strings or nylon? Nylon's not as tempermental, so it's better if you're going travel with it a lot, but to me the warmer tones of gut make it worthwhile to put up with a little extra tuning.
The best thing you can do is to go to a store (like Sylvia Woods') or to the homes of several people who own different harps. Play them, but don't forget to try picking them up and asking about the carrying case and the strings.