Between kickboxing and aikido, which one would let you get into better overall shape?

Probably kickboxing. I've done both, and both are excellent forms of exercise. Aikido "tends" to be a little less of a physical workout, but that can vary with the school. It does take muscles to roll and take a fall, but the throws themselves are meant to be done without having to use a lot of muscle. Kickboxing on the other hand focuses a lot on cardio and tends to be more nonstop so you burn more calories.