Aluminum block for the ability to repair it if anything catastrophic happened. Back when I bought mine, aluminum was my only option b/c the iron blocks weren't available new. The cost was so close it didn't make sense to go w/ an iron block anyway. If these newer iron blocks don't need any major machine work to be useable, then they'll certainly be cheaper b/c the aluminum blocks are more expensive now.
FWIW...the World iron block I have in the shop only needed to be bored/honed and we did a fresh line hone on it b/c I changed the main studs.

I prefer solid rollers. Buy good parts and you won't have any issues.