Id go with the steel. A friend has one of the aluminum ones, and the feed ramp part of the receiver is all chewed to crap. If you get aluminum, DO NOT use steel cased ammo, as that was part of the problem, and it broke the extractor. (the manual says not to use steel case, at least the one I found online. He bought it used and didn't have the manual) There's also some wear where the bolt handle slides, because the edge of the bolt handle wears against the receiver as you pull it back. If your buying new, either/or would probably be fine, but if it will be shot more than "average" steel might hold up a little better. The older AO guns made in West Hurley NY are not warrantied by Kahr, so if its used and one of the older ones, you have to source parts and fix it yourself if it breaks & not all parts are compatible (so Kahr tells me).
Not sure how they're made now, but one other small issue was had with the mag release spring on this particular gun. The spring is coiled around the release, and essentially "wound up" to provide tension. It would sometimes slip out of the hole that the end went into on the mag release itself when the release was pushed and the spring stretched. A little JB Weld to hold the end in place and there haven't been any problems so far.
Most of the parts are pretty large. The firing pin is a big piece of metal, the bolt is a chunk of steel, and the internals are all fairly large pieces of metal. The aforementioned extractor was the only really small weak part, aside from maybe the two recoil springs.
ETA: The Aluminum one should be just fine for your use, but if you get GI mags or ones not made by AO, check the fit. This gun has been run with GI stick mags, but they are loose and sit a little low. This causes some of the cases to catch on the edge of the feed ramp, and has battered the edge some over time (the chewed up part I already mentioned was the first 1/4" or so) and caused a couple hang ups until it was smoothed out.