which is best for middle layer(bll) whether dll or webservice in performance point of view

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performance

Hi, We are Creating a Webapplication which is a 3 Tier Architecture, Application-aspx pages, BLL-Webservice, DAL- webservice. Most of others use BLL as dll not webservice, so my doubt is , which is best, whether to use webservice or dll in BLL , Which one is Best in Performance point of view, actually our webapplication will be used by nearly 6000 users. So kindly let me know which one is best for middle layer