Of course being a tad bit biased on this one, I would have to recommend Dominical over Tambor because for all of the reasons you mentioned. Secluded? Tambor is more secluded than Dominical I would say. Dominical is a mere 35mins to Perez Zeledon which is the fastest growing city in Central America with plenty of shopping to satisfy even the most picky. However, I wouldn't recommend waiting to do your wedding attire shopping in PZ because that's best to be done before you come to CR. However the rest of your shopping can be done here.
I recently wrote an article on this subject that you mind find interesting and you can read it here:
Dominical, Costa Rica - Jewel of the Southern Zone