If someone is really serious and/or semi-professional, I see the virtues of having 2-3 15 bar 'harps, or even having a custom built model with interchangable bars. Wesley's question seemed to ask as if it was going to be one or the other.

I actually have two 15s and a couple other odd 'harps for certain uses, but if I don't feel like carrying several, I take just the 21, and I 'can' get to those 3 highest strings if I really need to.

The sound one gets with various types does vary, and if that is one of your main issues, do a lot of testing and experimenting.... doubling strings makes a serious sound difference, but I, as a leisure player, just never bothered to dedicate one 'harp that way.