What makes Shimano Ultegra Crankset or Cassettes better over 105, in Road Bike?

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I have 105 Shimano Crankset in my Road bike. What makes Ultegra better over 105? Just 100g lighter and looks better? Does it really improves the performance and how? I need to know the effectiveness before I upgrade.

I have 52/39T front and 12-25 rear. What combination I am suppose to use for flat road?
 
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