I'm a recent switcher - in April, a birthday present to myself, as it happened, and I love my MBP. There were a number of reasons why I switched; the superb and beautiful design, the sheer aesthetic appeal of the Apple products; the fact that they actually work well - I had bought an iPod over two years earlier and found it excellent and that iTunes knocked the socks off the ghastly Macromedia package found on Windows machines; the quality of Apple service - twice my iPod died within warranty, and twice it was immediately replaced without hassle or seeking refuge in obscure small print; and lastly, your concerns about stability and security, which I shared.
While with Windows, I had actually purchased a robust anti-virus, anti-spam package (most of the people I knew simply downloaded a free online version) and while that reduced the daily deluge of unwelcome visitors to a manageable proportion, it still failed to eliminate them entirely. Re stability, and security, Apple is excellent. The platform does not crash (I remember numerous crashes with Windows) and virus attacks are rare. Spam is negligible (yes a very tiny number slip under the radar but it is miniscule compared with Windows).
Re desktop versus laptop, my advice, for what it's worth, is to go for a laptop. They are portable and, these days, they are perfectly adequate in the power and performance stakes, too. I've used a laptop as my main working computer for around a decade (ok, I travel a good bit, but a lot of the time the laptop sits on desk and works as a desk top). These days, the screens on most laptops are excellent, - especially the MBP (and indeed Sony VAIOs) - and the sort of performance/speed/memory/screen tradeoffs for portability that one had to make until relatively don't really apply any longer to the same extent. That said, my personal recommendation would be the MBP, as I have one and it is excellent, but it's your call and it largely depends on what you wish to do with it. All of the Apple line up are excellent. Good luck with whatever you decide to get; you'll enjoy your birthday present for years.

MBP 15.4", 2.4 ghz, 2 GB RAM, 160HDD

iPod classic 30 GB