Both are valuable training tools.
Video, though, has severe limitations when it comes to actually learning new material. First, you're trying to get a 3D thing from a 2D medium which means certain perspectives will be distorted or not visible at all - even on the best of videos that have multiple angles covered. Second, no matter how many times you watch it the explanation and angles do not change. Third, a video cannot address tangential questions. Fourth, a video cannot make you feel the material being done correctly. Fifth, a video cannot give you any guidance on what you're doing right or wrong so unless your training partner is familiar with the material on the video, then it's a case of the blind leading the blind.
I think videos are great as reference material. If it's material I already know, then the video is great as a form of notes. Especially if it's video of me doing the material.
Seminars fall into two classifications. I'll use the term "seminar" and "workshop" to differentiate. A seminar is where an instructor comes in to a public setting and sets one or two pieces of pie on the table that everyone can sample - but they're not seeing anything remotely resembling the entire pie that the instructor has to offer. Or, sometimes, they get a quick overview of the whole pie but don't get to taste anything deeper than the crust. The seminar format simply doesn't allow anything else. You either get very little breadth with a little more depth or very little depth with a little more breadth. Seminars are great for getting exposure to a new instructor or system to see if one wants to pursue it further. If you go to enough seminars with the same instructor then you will, over time, gain some breadth and depth in the material the instructor brings out - but bear in mind that seminar material is almost always "public" material. It's the tip of the iceberg because the instructor never knows from seminar to seminar who will be there or what level people will be at. Occasionally, you get lucky at a seminar and end up with a room full of seasoned players and the instructor digs a little deeper because the room at large is more experienced. But that's rare and if you're not also at that more experienced level then most of it will likely go right over your head. Seminars are also good for networking. You meet a lot of people at seminars. Some are very good instructors in their own right and these connections may help you find an instructor you can actually train with on a regular basis. And the seminar circuit, like tournament circuits, is a type of community. Once you're a part of that community, you sometimes go to seminars just to visit with acquaintences/friends you haven't seen in a while.
Workshops are a little different from seminars. A workshop is when an instructor comes to a school on a regular basis and teaches a specific curriculum. The first workshop may be public or by invitation but subsequent workshops are usually invitation only and usually require attendance at the previous workshops (or the majority of the previous workshops, anyway). This format enables the instructor to really get into the meat of what s/he teaches and gives the attendees material that they can really work on and develop between the workshops. But this format is pretty rare.
I think all of these have their place as training tools but without a regular in-person instructor, it's difficult (if not impossible) to really develop any depth of understanding/application in martial arts.
Mike
