I've been a solaris admin most of my career, so obviously I'm biased. But if I were in your shoes I'd go with Sun hardware running Solaris. Here are the reasons.

Before getting to the specifics, the general hint I'd give is to figure out what you want out of your computing environment before buying anything. What levels of stability (24x7, or is some downtime ok), what software do you need to run, how important is vendor support (do you want them onsite if you have problems or is waiting for a part to be shipped for you to replace it ok) - and so on. Until you know the answers to those it is terribly hard to decide on the best platform for your company.

1. Overall to run a business I'd go with commercial over free. While Linux has made some big strides, it still isn't nearly as stable or well supported as the commercial Unix vendors. If I'm deciding what to run my business on that is the most important factor to me. Of course if you have minimal computing needs and having downtime won't affect your business too badly that isn't nearly as important. For example, if you're hosting your own website and having it down means customers can't contact you support and stability are critical, but if you just need to fire up the computer to process your month-end accounting and do some inventory stuff it really doesn't matter as much.

2. Solaris has the reputation of being the most stable of Unix variants. And when there is a problem Sun does a good job supplying needed patches for security and software fixes. In 4 years of working with solaris only once have I ever heard of a bad patch causing anybody problems. The guys I've talked to who admin other flavors of Unix (to say nothing of Microsoft!) have a much worse story to tell on that subject. . . .

3. Availability of software is better for Solaris than any other commercial Unix variant. I know of few packages which have AIX or HP/UX versions but don't have Solaris, while there are plenty which are Solaris only. Of course this one depends on what you'll need to run, you should decide that before your OS/Hardware decision is finalized.

4. Finally, Sun has been coming out with some good machines for small businesses lately. You can get 2-4 processor boxes dirt cheap compared to what a Solaris box (or other commercial Unix) would have cost just a couple years ago. Some of them are well under $10,000 and thus for only a bit more than some intel box running Linux you can get a Sun box running Solaris with the much better level of stability and support that gets you.

Like I said, just my opinion - but an opinion with some good reasons behind it. Take it for what you will.