I have never been to the National, but from what I have been told there is not much hockey. Nothing compared to the expo. I am hoping to attend the National this year (my collection is scheduled to be on display at the PSA booth, but that may change), but I don't think there is much hockey.
I have been to three Nationals (Cleveland x2 and Atlantic City) and while they were fantastic events, the one disappointment was the lack of hockey dealers. In fact, the four best were Jason Chartrand (though he tends to set up with a fraction of his material), Len, Harvey and Jason Martin. In other words, all the big Toronto Expo guys are all the best hockey dealers at the National. There are a handful of others, but if you are going to Baltimore in search of a fresh and bountiful supply of hockey cards you will likely leave unfulfilled.
I went last year - it was in Cleveland. Not much hockey to speak of - maybe a couple dealers specializing in hockey and then odds and ends at a few other tables. It was my first National in years and it was well worth the trip! Loved looking at the old baseball cards and memorabilia and specialty dealers (there was one guy set up only selling Japanese baseball cards - "menkos"). Didn't buy much - prices were too high - but I would highly recommend if you had the chance (maybe not if you're just looking for hockey because you'll be disappointed). It's a couple times larger than the Toronto show.
Better get insurance for your collection before taking it on the road. I'd be too worried to show my cards at the National after the one a few years ago where one of the baseball card dealers had some of his cards stolen. I think it was the 2007 show?
Definitely know what you mean. I already have insurance on my collection, which includes travel and shows. Also, if I do show my collection it will be at the PSA booth and will be insured on their policy while in their possesion and they will have a guard on duty as well. They did this for me when my collection was on display at their booth at the Summit show in Edmonton.
I have been to about 5 or 6 National shows. I agree that there is more baseball than hockey, but I have seen vintage hockey at these shows. You never know what will show up there. It is the first time being held in Baltimore, so that may or may not bring new dealers in.
GO TO THE NATIONAL !!! You will have a fantastic time, regardless of how much or how little you find. I can't stress enough how enjoyable the show is, especially the social aspects of the hobby.