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I'd go with
www.mybrand.com/store/category1/subcateory
www.mybrand.com/store/category2/subcateory
www.mybrand.com/store/category3/subcateory
...this keeps everything neatly organized for both SEO and the UX (not to mention your content management).
I've been building professional ecommerce websites for 12 years.
Fred
Seattle Web firm w/ over 400 clients looking to add a few contractor or permanent staffers to our downtown seattle office -we have a great staff and really good coffee -does anyone know any good people that might be interested?