Optimum number of clients for service bus

Hi

I'm new to service bus and kind of analyzing its capabilities. We're having a debate over whether we should have a front end component (most probably a REST service) sit in front of the service bus and off load incoming requests from clients OR we should allow clients to connect directly to service bus and long poll.

I'm looking for opinions and best practices. We could have up to 20000 clients long polling service bus. How does either approach affect scalability of the solution? What other things should be considered?

Appreciate peoples input on this. Thank you!