I have had a Chicago Petit for 2,5 years. It was a wonderful harp; big and warm sound and not so big. There was only one aspect I did not like: the smaller space between the pedals. When I played with pedals it was always a problem taking lessons on harps with a wider spacing than my harp at home. That is the main reason I bought I Concertino in december 2012.
There is not so much difference between the sound of the Petit and the Concertino. The Petit has great sound for its size. But playing on a harp with “normal” pedal spacing makes it worthwile to buy the Concertino. Then you have 47 string in stead of 40 too. I hope to play La Source (Hasselmans) in the future and now I can do that without limitations. Here in Europe there is no bounty program because of the tax. So I had to pay several thousands of Euros extra to get te Concertino. I have not regret it for a moment.
Good luck with your choice!