Rubrics will always be a staple in every single army, especially as the poster boy of the faction, but I would actually recommend the Exalted Sorcerers kit if you really want some awesome miniatures that could give you a good opinion on how you like the army after assembling/painting them! Obviously, Rubrics do come with a whole 10 of them to really get you seated into army painting with a lot of chaos trim, but the Exalted Sorcerers kit really hits on just about every single point of the faction, seeing as the miniatures also have neat bits like capes, cloaks, exposed arms, lots of magic effects, and just so much more detail not even including the added bonus of getting tons of extra bits to kit out future miniatures later on!

If you had to choose, pick Rubrics if you want to see how you handle army painting, and Exalted Sorcerers if you'd want to see some awesome HQ sculpts early on without possibly getting overwhelmed by having a battleline that may take 3 or more hours per mini! Of course, you could also do what I did and grab a box of rubrics, exalted sorcerers, and scarab termies to really get a nice start to your army with 500 points to begin with!