You should talk to GrowlTigga, if' he can still be found on these forums. By all accounts (namely his own
) he is about your stature and is has taken several martial arts. I know he's trained in Shotokan karate, like me. I think he did full contact as well which is, frankly, utterly insane, unless you happen a reconstructive dentist touting for business.
I'm of a similar size to you, but without the bodybuilding, and I find Shotokan to be very suitable. It's very much a "brute force" style: Unlike other martial arts / karate styles where the aim is to deflect, unbalance or throw your opponent, shotokan is all about hitting your opponent very hard so that he falls down and doesn't get up for a while. It may not be the most energy-efficient way of achieving your goals but it has a certain simplicity that appeals to me.
What David Gervais talks about is valid, and even Shotokan teaches you to take advantage of an unbalanced opponent, but I'm very surprised a karate instructor allowed himself to get pulled off balance like that, although if you were doing something other than what he was expecting in a demonstration then I guess it could happen to even a good instructor. One of the first things you learn in Karate is stance, and with proper stance you should never over-reach or put yourself off-balance. It's fundamental to karate.
I tried Judo a while back, and enjoyed it. Although it is utterly unlike karate and punches/ kicks are completely disallowed I found that the stances and balance I had learned in karate (combined with my considerable mass and reach) made it hard for even experienced judo ppl to throw me if I really resisted them. I only did about a half-dozen lessons (including an introduction to something called Sambo, which seems to be similar to judo but with less restrictions) but I would be glad to do more of it. There's something very satisfying about picking a person up and dumping them in an undignified heap on the ground- far more so than scoring a punch/ kick on them in Karate.
In terms of self-defence, I think a combination of different styles is the best solution. No martial art is really good for self defence until you've got a good amount of training behind you (say, a year or so, training 2 or 3 hours per week with a good instructor) but they can work. I've never had to use karate in a 'real' situation I'm fairly confident that it would be useful to me if it ever came to that. The main thing though is intent- In a 'real' fight I would pick a determined newbie over a black belt any day if that black belt didn't have a real determination to hurt the other person.