Did you actually ride the bike? Did the first LBS person explain to you why she thought it was too small? If I remember correctly, the 15 inch is a woman's model, and it is the smallest size. It would fit me quite well (I'm only 5 feet tall), and she may have thought you should go with the 17.5 inch model.
You're the customer, but if you purchase the 15 inch bike, you may find that it is too small. Also, I think the Tiburon is a step down in the hierarchy, so you'd be getting a lesser bike.
I would definitely recommend riding the Zebrano and see how it feels. Try to make it a longish ride, around an hour or so if the LBS will allow you to do so.
East Hill
__________________
___________________________________________________
TRY EMPATHY & HAVE LOVE IN YOUR HEART, PERHAPS I'LL SEE YOU ON THE ROAD...