For looks, DEFINITELY Sony. Sony laptops are freaking sexy. (I'd suggest the N series. They have AWESOME colors.)
I've owned laptops from both brands and I like Sony a lot better. The screen is really bright and clear. With Dell, the screen is ok. Not as bright or pretty.. And I really don't like the keyboards on Dells (7 keys fell off my first Dell laptop).
Sony has the Memory Stick thing, which is really bad because only Sony products use Memory Stick.. Dell has those 5-in-1 media card readers, which are really useful. Sony has it too, but the laptops with them are more expensive.
Sony laptops are of better quality than Dell, but they're a lot more expensive..

If you want a really good laptop, go look at Toshiba or HP. I just got an HP Pavilion Entertainment laptop today and I love it! The HP imprint finish looks really great (and it's so shinyy!). You can get a good laptop for cheaper with HP, and the same goes for Toshiba. But Toshiba's laptops aren't really that pretty.