Canon EF 70-200 f/2.8 with or with IS

[peterborough_photography]

Im sort of in the same boat I dont know if I should buy

• Canon
  EF 70-200mm f/4.0 L IS USM

OR

• Canon
  EF 70-200mm f/2.8 L IS USM

Price wise in the UK its about £400-500 dif





Any views from anybody that has used both ????