depends on how you feel. i went for the most expensive comfortable looking one in mothercare and im not sure theres much difference really...dd just rolls onto the edge of it anyway. ds just had a standard one and that was fine (and after 7 years still very bouncy and used frequently in our garden in summer for tent picnics!) sorry im not much help really am i?
First time we had a M and P natural fibre one and even though ds was not a heavy baby , it seemed to wear more than the Mothercare Sprung one we had for dd next. Only thing I wished I'd had for her was a removable washable cover as she was a sicky babe.
we got the most expensive sprung one from mothercare (hooray for generous friends of my parents!) - i think it was about £80. ds used it 'til he was 2.5 very happily & i intend to use it for our next baby.