Hello Jusroc,
I'll offer my insight in hopes of guiding your path to learned self-defense groundwork.
My grappling training is from the Tagaki Yoshen Ryu, and I considered myself an excellent grappler. Until I grappled with some high level GJJ practioners-they owned me- but I was working within the context of the competition rules. In self-defense there are no rules, no ref, and no illegal techniques. That said, barring the more dangerous techniques, GJJ is that much more absolutely solid in it's basic principles and tactics, however as Tony Dismukes explained above, the strategy is taught mainly from a sport-fighting perspective.
The key differences, as I see it, is that first of all, on the street you do not want to go to the ground. you want your attacker on the ground, you on your feet(or one knee), unpinned, unentangled, head up eyes alert and ready to move quickly in any direction. you want that, but you may be on the ground with your attacker on top of you from the first anyway, and you must have the basic do's and don't's of groundwork under your belt. GJJ will provide that, but strategically it is a bad idea to seek to put yourself on the ground, depending on your skills and the circumstances, of course. I have known people who were so good on the ground that they could take out multiple attackers easily from below, but if you are that good, you already know that.
in the ring, taking someone down and positioning for an arm-bar is a good strategy. you can afford to rest on your back and let your opponent wear himself down until the right moment to seize advantage presents itself. on the street there could be a second attacker, or the first could reveal a weapon. in Takagi Yoshen Ryu, the attackers arm should already be broken before he hits the ground, and immediate escape is the strategy. Any take-down or submission hold that ties you up and entangles you with your opponent's body makes you vulnerable and should be avoided.
