I know, the question is a bit bogus, as one should be comparing arts such as Karate and Aikido merely so as to make them compliment each other. Thus, with that statement made, let us discuss how the fist should wiggle into the glove. No pow and bam, just an honest, up front viewpoint for your edification and enlightenment.
Karate is supposed to be straight line, and Aikido is supposed to be the circle. Yet, if on looks at Karate, let alone the martial arts, one will see that perfection of geometric figure is loose, at best. If Karate could actually adhere to the perfect line, if one examines how the bones, joints, muscles and so on function, the body would probably explode, or, at least collapse upon itself.
And, on the other hand, if Aikido tried to adhere to the perfect circle, except in the most theoretical of classes, the art would not work. And, to be honest, aikido is not your best art for down and dirty combat. While Aikido is pure and wonderful, and can evolve the practitioner to high levels, one should use a martial art like Karate to enter the fight, then apply aikido.
Think about it this way, a fight is the collapse of distance. The length of the arm, and the circle being made, the lever of the extend arm is too long. However, if you want magic, use karate to kick, punch, find yourself on the inside as the distance collapses, and then use a shorter lever.
Instead of doing a two step and trying a three foot arm circle to a wrist twist, try a hard middle inward block, slide in, maybe with a short fist or elbow, then turn the waist. As you turn the waist, bring the arms up to a short position, lever the elbow, shove your shoulder in and pull. Go ahead, try this scenario with a friend, watch a little youtube to get the motions, and you are going to find an instant blend of karate, even the hardest of karate, like Shotokan, and with the purest of Aikido, even the soft of Morihei Uyeshiba.
Now, the above being detailed, the big weakness of Karate is that it is limited, in most modern formats, to destruction. It has been tailored for tournament, gloves put on for more violence, and given over to fighting for fighting sake. But while there is an art to destruction, the true art is in control.
Thus, a study of Aikido, with the advices I give you here, will enable you to confront the fiercest violence, and transmogrify that violence into the most magical and wondrous of techniques. You kick, you punch, maybe soften the fellow up, then you simply embrace the arms and learn how to go with the flow.
A last word about all this, don’t mistake the throws of judo or jujitsu for Aikido techniques. While techniques of the ju variety are wonderful, we want to move from hard to soft complete, and a certain amount of hard is still involved in ju techniques. That all said, I wish you the best with your new art, and a new art it is…karido…aikate…your choice.