It's always the client who pays money, so states the final
decision, but are you his consultant? If so have you
make his aware of how much money he'll save/loose and
when deciding about his app. server?
I admit there are many reasons that may encourage
using BEA's Weblogic, but take care if this is really
the correct solusion ;)
BTW, about migration  from oc4j  to Weblogic, if you're
using versions of weblogic  prior to 6 you're in a
bad trouble. Since Weblogic 5.xx doesn't accept .ear
files.
I recommend 6.1+ , if you're having .ear files you need
only a bit changes in your application.xml and i think
web.xml (i'm not sure of the latter).
Also you need to replace oc4j's specific deployment
descriptors with weblogic's which needs studying its
DDT. But if you're building from scratch, you can use
weblogic's console to deploy EJBs, Web apps (war files),
and even j2ee applications.