If I were you, I'd use linux seeing as though it is free, and although Solaris has become quite affordable, it still isn't free as far as I know. In order to learn, go buy a popular distribution such as Red Hat or Suse, and get a good book on the subject. I like Running Linux, and it's an O'Reilly book. I used to use Windows Me and I got so frustrated one day that I completely erased it off my hard-drive and installed RedHat 7.0. While, it was a little difficult at first ( I had trouble configuring my Netgear Fa311 card), it has been great ever since. My computer hasn't froze once. One thing that you'll find easy is that these linux distribution come with a nice GUI, so it looks similar to windows, so you should be comfortable enough to get around. I'd recommend using the shell instead, as you tend to learn all the commands a lot quicker. Good luck, with your decision.
i just started with Linux last year. i chose Redhat 7.0 and i think it is fairly simple for a beginner. i have not tried Solaris, so cannot give you a comparison between the two. Redhat just came out with 7.1 that uses the new Linux 2.4 kernel. it also provides drivers for USB devices and more printers than the 7.0 version i have. check out the Linux website for details