Re: aikido vs jiu-jitsu
my limited understanding is that
Aikido seems to specialized in the take-down phase, or the clinch, area of no-weapon martial arts;
most Kara-Te (or karate/TKD) specialize in the contact phase, the stand-up;
Jiujitsu (esp. BJJ) specialize in on the ground while Judo practice evenly on take-downs and on ground.
if you're "playing*" jiujitsu than you must work on jiujitsu to counter.
Judo or Jiujitsu's a lot of fun and a great workout.
*it's either a game (with judges and prizes) or it's Budo (of life and death matter)."