Kent Enfield

As someone who practices both kendo and iaido, I'd say do them both. You may like both. You may like neither. If your main goal is to improve your aikido and you can only do one, I suggest that you do kendo.

Now I think I should respond to Craig and Nathan's posts, as it seems they probably havn't actually practiced kendo and there are some, um, misconceptions in their posts.
If by "offense. offense. offense," you mean continuous attacking, this is true at lower levels. It is part of the teaching methodology of kendo. It is about learning proper seme, usually translated as "pressure," but I prefer "assault." The ultimate goal is to develop formidable kiseme. This accomplished by first starting with continuous attacks. This teaches one to recognize suki as well as a crude method to seme. From this one learns to not just to see and seize suki, but to create them. This is sometimes described as waza-seme, pressure/assault with techniques. From one learns more subtle technique and enters into kisaki-seme, assualt with the sword tip. This is the stage at which kendoka seem to constantly feel each other out, jockeying for position with their sword tips constantly moving. From this, they move on to kiseme. This what you see those old guys do where they just cross tips, stand there for a little bit, then one steps in and strikes the other.
I'd call whacking a gross mischaracterization of kendo technique. Even for beginners, there's a lot going on. But, yes, in competition, points are scored by striking certain points of the body with a shinai. It needs to be done with the correct part of the shinai, with correct edge alignment and motion, making use of ki-ken-tai-itchi (everything in unison), while demonstrating zanshin.

You're mixing Japanese and English. It's either AJKF for All Japan Kendo Federation or ZNKR for Zen Nihon Kendo Renmei. The iaido seiteigata are in no way required for kendo students. They're required for iaido students, naturally. The Nihon Kendo Kata, the 10 paired kata, are required for kendo students. Yes, there are kendo students who wouldn't know what to do with a real sword, though they'd certainly know the tsuka from the kisaki. There are also plenty of kendoka who would be fearsome with a real sword, and have never done iaido, or any other sword art.
This depends entirely on the dojo. It hasn't been true in the dojo where I've practice.
Iaido kata are about killing other people, ideally before they can even get their hands on their weapon. While the low level kata are all defensive, if you count things like "sensing hostile intent" as a reason to open someone's neck, the upper level kata are for situations like sneaking up behind someone in the dark, pushing through a crowd to cut someone down from behind, and hiding under the porch to literally stab someone in the back. Obviously, most practitioners find something else of value in the practice.
Finding an iaido school that does more than the solo kata with occasional practice of some of the two person forms by advanced students is very rare indeed.
Both of these are very rare in North America, though kendo federation jo is growing in Canada.
True. Again, visit both. Try both.
This is completely untrue. Even the most competition oriented kendoka is going to do more than tap things with a stick. I suspect Nathan has never actually received one of those "taps."
Iaido will generally teach you to cut air. It generally won't teach you when and where to cut, as your imaginary opponent is always in the location you want moving at the speed you want.
Never actually done kendo, have you? At least not for any significant length of time.