To build on what v giri said, it's really a difference between generalization and specialization. The IBM exam (I took 483, but I imagine 484 is much the same) is a general
J2EE exam - it covers the concepts, but doesn't go too in depth into any portion of the material as it needs to cover the whole of the enterprise edition.
From having taken (and, lamentably, failed) the SCBCD, the Sun exam is concentrated on the EJB portion of the code. Sun can do this, since they have a seperate exam for the web components, and it allows for more exacting questioning.
Now, as for which will benefit you more, this is a value question that generally, only you (and perhaps your employer) can answer.
The IBM Solution Developer certificate is certainly useful to have if you're using IBM products. Most of the certs involved in that - IBM's UML (486), J2EE (483/484) and Sun's SCPJ2 - are general enough that they'd apply to any job... but the certificate for WebSphere, which involves installing, tuning and administrating WebSphere, probably only benefits if you're working in a shop that uses Big Blue products. On the other hand, Sun's certificate is as 'vendor neutral' as it can be for the technology (as in, not depending on any given appserver), so as a more general proof of potential ability to leverage the technology.
Hope that helps, even if it's a bit verbose.
