There are a large number of threads on this topic but for me I decide based on the following:
Westin: Smaller hotel with less of a Vegas Zoolike feel. I personally like this but others complain that the casino is too small and it just doesn't feel like Vegas! Westin Heavenly bed a plus but you are likely not to get upgraded.
Aladdin: Big Vegas hotel feeling. More likely to get upgraded but you might also have to deal with ongoing renovation/rebranding to Planet Hollywood issues.
I prefer the Westin because I usually go to LV on business and like the quieter feel.