The difference is as follows:
Ultimate is an API SM grade, where as the Premium is an API SG grade.
There's a rule in reading lubricant specs and what they mean in layman terms.
API grades starting with "S" means SPARK which is the main characteristic of a PETROL engine!!! API grades starting with a "C" means COMBUSTION which is the main characteristic of a DIESEL engine.....
NOW, stating it very clearly that both these are meant primarily for PETROL engines!
How you read these grades is fairly simple. The higher the alphabet goes on the alphabet table, the better the quality of specs and additives or base oil it is blended from. therefore, SM is much superior than SG !!!!!!!!!!!!
Coming down to the viscosity index, where here is 10w40: this means that between what temperatures on the viscosity table can the lubricant hold it's true properties to clean, prevent and protect. Both being the same here in this case we'll just have to see the API specs alone to determine which one's better.
Hence: the conclusion one can derive from this long lecture is SGO ULTIMATE (SM 10w-40) is far better than SGO PREMIUM (SG 10w-40)!
also, selecting your lubricants be it shell's or PSO's or Caltex's - one should know as to what the engine specs are and what the OEM recommendations are with respect to the API grades and viscosity - so select your lubricant's accordingly.