I would go for the LPIC certification first as it is broader and will be of more value to you job-wise. If Oracle should decide to actively develop and/or support Solaris further and the market is in need of Solaris admins and that is still what you really want to do, then you can also go for the Solaris certificate.
But the Linux certificate seems a saner choice to me at this point. Personally I also think that Oracle plans on further developing their Red Hat clone (Unbreakable Linux) instead of choosing Solaris as their main platform.
Anyway, good luck!