I would go with the dell. But then I find OSX to have the most awful GUI i've ever used in a computer. Having used the dells my friends got recently I found them to be really well put together and great value for money.
If you really feel the need for speed and security then go for Linux.
If you're looking for stability in an OS then use dos. Vista, OSX and Ubuntu Linux (i don't know about the other linux distros) all tend to have problems, bugs, glitches and crashes at about the same rate in my experience.
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