A lot of local areas have great cons--however, they're not going to attract the large number of professionals and editors that a national con will.
Every year, one local con earns the right to transform into 'Worldcon' for that year. Though the con itself doesn't change much, it gains the power to give away the Hugo awards, and generally everybody agrees to go there instead of somewhere else. So, that local con--instead of its standard two or three hundred people--will balloon to five or six thousand people.
The same thing happens for 'NasFic,' the big, national con they hold in the states every year that the Worldcon is outside of the country. World Fantasy is kind of similar, but it doesn't 'transform' a regular con--it is put on with a specific theme by people who organize local cons.
I'm not sure about Westercon, which is the other big national convention.
Cons like DragonCon are completely different. They're media cons, and are for-profit (Worldcon, World Fantasy, and Nasfic are all volunteer, non-profit cons.) Dragoncon will have huge attendance numbers, and will be filled with booths and media presentations, as well as have tv/film stars as their guests of honor.
Local cons, like Philcon, are probably very fun--but they don't have the networking power to draw people from all across the world to attend them. They're usually much cheaper, but they don't have the same kind of programming and the like.
