One is from the East the other is from the West. Thats the only difference. Appearance
There is no difference between the two types of Shellos (eastern and western). They have the same potential Abilities (Storm Drain or Sticky Hold), the same potential movesets, etc.
The only difference between the eastern and western Shellos are their shapes (western Shellos have three knobs on their head, eastern Shellos have two "horns") and their colors (western Shellos are pink, eastern Shellos are blue). The variance extends to the evolved form of Gastrodon as well.
Also, western Shellos, which appear earlier, can be caught at a much lower level than the eastern Shellos when either is caught on land. On the water, they are at approximately the same level.