For me, the deciding advantage of using RHE linux over the other two is that the dependencies are managed with rpm.

Also cross-compatibility of versions of different software (e.g. version of gcc library versus RDBMS version) is almost guaranteed for RHE-Linux (irrespective of the point about rpm) whereas for Sun Solaris I am yet to find how to assure that issue. I don't know enough about such cross-compatibility for FreeBSD to compare it with the other two though.