There is absolutely no reason to choose a 15-chord over a 21-chord for a chromatic harp. The 21-chord lets you play in more keys and has the added benefit that you use the chord-bar buttons are arranged the same for all keys, so you can quickly learn to play in any key without having to look at the buttons.

Also, don't limit yourself by looking only at Oscar Schmidt harps. If possible, try to play as many different harps as possible (including the various Oscar Schmidt models--they vary considerably in sound). There are luthier harps that are not much more expensive and which you might be more satisfied with. I've been very pleased with my Lumbert harp (http://www.lumbert.net/)

If you aren't familiar with the Cyberpluckers, an online autoharp "community" full of advice of all sorts, go to www.autoharp.org to subscribe. Ask your question there and you'll get lots more responses. They'll also be able to tell you where you might get to try out some of the various harps, possibly near your area.