If to select between this two I would suggest Mac. If not to limit the selection of brands only to dell and mac - I would suggest Asus, due to its price and functionality.
Both choices are pretty decent in my opinion. If I were to chose I would probably go for Mac. They're relliable and they simply get the job done, whatever it is. On the pros I suppose that Macs have wider support and compatibility with software and a solid userbase that proves to be helpful at times as well - especially when something goes wrong (from my own experience).
it all depends on what you want to do. If you are looking for a good game box with solitaire and hearts, go with dell. If you wish a apparatus that can run 3rd apple countries and seamlessly affix with any coffee boutique network, go with a Mac. And with Office 04, you can do anything that dell can do, but 20 times faster and a whole lot cooler.
Dell, since most programs are made to run on PC. Mac is pretty problematic at times. Its only use is it's so called "invulnerability to viruses."