First of all I'd point out that Linux is freely available to install on a sacrificial box at home and that the concepts (if not the detail) carry over between both fairly well. I initially learnt to use Linux because I found it fun and found work with an ISP through helping out on a LUG mailing list. Subsequently I was hired to a position which exposed me to Solaris and the differences, while many, are well documented (the
unix rosetta stone site was invaluable initially). So I'd advise tinkering with Linux at home and perhaps certification (the RedHat certification is good but pricey, the LPI is worthwhile as it shows you have done enough to make yourself useful but RedHat is the de-facto industry standard. CEntOS is a pretty exact copy of RedHat but free to use.
1) There are plenty of both
2) I'd go the other way, but by and large yes, unless you have the misfortune to encounter HP-UX
3) Solaris is probably the most common version of Unix
4) Yes, but again I'd recommend Linux for self training.
5) the same general principles apply across the board
6) Curiosity and access to a Solaris box
7) Yup, role dependant.
One final note, which may be entirely subjective. I have noticed that Solaris use is reducing and Linux on the increase among our customers globally, with North America and N. Europe being the Solaris hold outs for now. HP-UX has almost completely disappeared. I haven't seen an enterprise SCO box for 3 years and BSD shops do their own thing and don't need our help