I know why you feel one sign is better than other. They both are great signs they certainly have different characteristics. Due to limitation of the forum I cannot go in too much analysis of each sign, but even if you read more on astrologymag.com, you will get more information.
Each sign of the Zodiac is related to an Element. These elements are Fire, Earth, Air and Water. The list goes as below.
Sagittarius has fire qualities. Which makes them outgoing, aggressive. If they have difficulty in expressing themselves, they can become selfish and risk takers. However they are very motivated, focused and confident.
Pisces is a watery sign which makes is very emotional even if they dont display their emotions. Pisces native are quite resourceful, easily adapt to changes. They are caring, artistic but like to live in a dream world.