I have been reading the different specs for different 50 cal rifles and conversions.  The ALS is heavier (fatter barrel).  Are there enough advantages to the heavier barrel to offset the disadvantage of the heavier weight?

Listed are just a couple weights

ALS weight 35 lbs (upper only)
serbu weight 22 lbs (complete rifle)
AR50 34 lbs (complete rifle)

I am really interested in the ALS conversion, so please discuss this option as much as possible.

Well, the aded weight will soak up more of the recoil.  It feels no worse than shooting a 12GA shotgun with the muzzle brake.

If weight is a concern of yours,  you can always get it from ALS in the "light gun" configuration which means he flutes the barrel.  

Either way, be prepared to wait.  Darran has at least a 6-month waiting list for his uppers.  He is a one-man show which explains most of the wait...