Whatever suits your kart best.
We have had numerous discussions on the subject but the answer lies in how you have set up your kart and the track on which you are operating.
Nominally Mags are lighter, stiffer and conduct heat better than allys. They are also more prone to break, which is really relative as to hit them hard enough to do so is going to damage if not break something else on the kart anyway.
Again, nominally, a light rear axle will have less mass and therefore accelerate / decelerate faster, but in fact one suspects the difference is only detectable in a Lab. On the track, with all the other variables, it probably isn't going to make the slightest difference.
There are certain circumstances where a stiff wheel can be an advantage, coupled to the appropriate axle. It depends on your driving style, weight and the type of corner and the number of that type of corner on the track.
Equally heat dispersal can be a blessing if you are running your tyres 'hot' but a curse if you are running them 'cold'. Are you a sufficient driver to run and implement a tyre strategy on a race day? (if you are asking the question, probably not.) However, given that the TKM / Tyre combination is a low grip, hence lower heat input, it is possible that alloys are better. However, it is probably cheaper to paint mags white inside and out, but not on the bead recesses, rather than replace them with alloys. (this cuts down the heat absorption and radiation through the rim)
So basically, it doesn't make that much difference, though you will find people who swear that the change onto one rim or other found them that extra n'th of a second per lap.
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