I have standard carbine handguards on my 16" AR, but I was wondering if the Aluminum Rail Type would be better for dissapaiting the heat.
The 2 PC Rail Types don't have the holes on the top, so there is a possibility that they might hold more heat in, but they also have more holes in them than the standard ones.
Would M4 handguards be better?
You basically have the answer to your question. 2 piece Carbine handguards will retain more heat because it has lesser holes for ventilation. More so with M4 handguards because they have double heat shields. But since its construction is made of plastic / polymer, heat transfer on the handguards is more bearable to the touch compared to aluminum handguards.
On the other hand aluminum handguards DO have more holes on them giving them more ventilation. But aluminum conducts heat better than polymer / plastic. With the use of rail panels, this makes it more "comfortable to the touch". Better yet, get a vertical grip to go with it.
Hope that helps.