if you have ever actually run Solaris, most would say to go linux (as would I). solaris can be a complete PITA to get running properly, and once running, getting native development packages can be quite expensive (think thousands for sun's compiler suite and toolchain). sure, you can use GCC, but you pretty mush have to distribute glibc and any other libraries you use with whatever you develop. There is Linux Binary Compatibility built in, but anything that requires kernel modules will fail.
Solaris, I'm sure, does have SOME uses over linux, the main thing i can site, is that, for unix purists, the "true" UNIX kernel is a draw, but a good BSD or properly configured Linux system is just as good in my experience for almost all tasks.
For a bit nicer (IMO) UNIX, try HP-UX. the drawback is you will need PA-RISC box or (for really old versions - 10.2 or earlier), a 32-bit 680x0 system with MMU.
