The stock market is presenting all kinds of buying opportunities that are making me interested this week, but the one dynamic that stands out most today is the dramatic difference between the performance of two companies in my portfolio that sell high tech products that both do essentially the same thing ... though in very different ways.

Accuray (ARAY) is the maker of the Cyberknife, a targeted radiation machine for cancer treatments that has been called the "Cancer Blaster" by newsletter touts who enthusiastically recommend the stock. Though useful for all kinds of radiation treatments, the argument for this more advanced machine is that it can track moving tumors and spare nearby tissue -- making it particularly popular for prostate and lung cancers (since the lungs, you know, move, and the prostate is in, shall we say, a very delicate area of the body).

Intuitive Surgical (ISRG) is the maker of the da Vinci surgical robot, whose core market is prostate surgery (though other surgeries, including mitral valve heart surgery and gynecological applications, are growing fast). The strongest rationale for fast growth in the use of this robot a couple years ago was that prostate surgery with the da Vinci spared those same "delicate areas" near the prostate in a way that open surgery often did not ... though it certainly also has other advantages over open surgery, and, depending on the surgeon (and who you ask) over traditional laparoscopic surgery.

Accuray is up close to 10% today, Intuitive Surgical is down about 8% as of my last check. Why is that?

Well, it's certainly true that ISRG is much more steeply valued, being one of the top performers on the Nasdaq for all of last year, and there are huge numbers of shareholders, myself included, who have a cost basis several hundred percent below the current price -- that makes these shares a target for profit taking when shareholders start to panic that we're entering a recession and/or a prolonged bear market. Even the most enthusiastic shareholder would have a hard time arguing that ISRG is a bargain, even down $70 from its high, with a forward PE of about 60.

And Accuray is, on the face of it, much cheaper -- this much newer public company (IPOd over the summer last year) has much lighter penetration in the marketplace, and has some competitors who offer somewhat similar products (as opposed to the da Vinci, which has the multipurpose surgical robot space more or less to itself).