You really, really, really need to be a bit more specific, Brian. What exactly do you want to know? On a general level, you might want to pick up a guidebook to do some research. People here will be helpful with specific questions...but if you don't have a vague idea what sort of info you're looking for, I'm not so sure we can be of much help.
As far as the cities go, Barcelona has become something of the darling of Europe. Its architecture is stunning, its nightlife is vibrant, and it's one of Europe's great cosmopolitan cities. In fact, Barcelona often feels like more of a "European" city than a Spanish city. Madrid, on the other hand, is quite a different story. Sure, it might not pull you in at first glance. It doesn't dazzle like Barcelona, it doesn't charm like Salamanca. Plenty of people never fully take to it...and admittedly, if you're visiting the city now, it won't exactly put its best foot forward: half of Madrid seems to be under construction, and you'll have to deal with the sounds of jackhammers pretty much any hour of the day.
So why go to Madrid? For starters, there's a reason Madrilenos are known as gatos ("cats"). These are notorious creatures of the night, and if you haven't gotten into the habit of dinner at 11, drinks at 2, and dancing until the wee, wee hours...you will. Madrid's nightlife is spectacular, and the city really comes into its own once the sun goes down. There's also plenty of eye candy to go around, since Madrilenos are incredibly fastidious. After a couple of days, you'll be able to pick out a "pijo" from a mile away (you'll figure it out once you get there).
The city's also home to one of the best collections of art in the world. Between El Prado, Reina Sofia, and the Thyssen, you've got three spectacular galleries within a short walk of each other. Even if you're not into art, it's worth an afternoon of strolling through El Prado, just to watch the hordes gawking at Velazquez's "Las Meninas."
Before you know it - even if you don't know when or how it happened - there's a good chance you'll fall for Madrid. But Barcelona is praised for good reason. I suspect you can't go wrong with either. Just depends on what you're looking for.
Chris
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