As a general rule, you should always strive to get the SSR weapons for the elements you plan to use. Even the SSR Defenders are worthwhile because you gain a lot of HP for a small sacrifice in damage compared to an SR Assault. If you gimp your grid by forgoing SSRs, you'll regret it later. A missing SSR eidolon is generally replaceable or recoverable. An SSR weapon is not; your grid can end up permanently gimped.