In a recruiting website, should I force a choice between one table for jobseekers and employers or 2 separate tables?
From a user experience point of view, it would be much easier to support only one "login method" without having to select which type of login a user wants to do (e.g. a dropdown for job seeker/employer). This would also mean all usernames would need to be unique. 

If you separate the tables, you can still have optimal queries, they just might have an extremely small join associated with them (or just create a denormalized view of both job seekers &amp; employers and you have best of both worlds!).

I think the real question you should ask is:  Do job seekers &amp; employers have fundamentally different data associated with them or is there a large amount of overlap?