5.3 Cast Iron Block (LY5, LMG) vs. Aluminum block (LC9)

Hi guys.  I'm looking at ordering a 2010 Suburban and am a little confused about some engine choices.  There is a 5.3 iron block that's standard in the 2wd,  and a optional flex-fuel version of the same iron block.  In the 4wd there's a aluminum block version.  The iron blocks are slightly higher rated (10hp) but are there any reasons or concerns with either version?  Any problems with the flex-fuel versions? Thanks for your help.  

The only problem in flex-fuel is the availability of fuel to flex. E85 is not available anywhere in California. So if your a Cali resident then your stuck till the gas companies get there head out of are @sses.