Boilers have become so efficient nowadays that I bet you wouldn't see much difference between a combi and a condensor in terms of the cost for the thermal energy required to heat all that water.
We upgraded from an old-fashioned iron core boiler to a modern condensor about 5 years ago and our gas bills dropped by about 30%. The boiler has been reliable and trouble-free, in contrast to the problems some people seem to suffer with combis if these and other fora are anything to go by.
I definitely prefer having a cylinder full of hot water - it can be supplemented by the immersion heater and is fed water that has been pre-heated in a supplementary cylinder by my simple solar panel. The airing cupboard is a nice warm place to store towels and sheets and ensure kids' clothes are nice and warm and dry. As somebody else wrote above, I wouldn't want to run my heating system at more than normal gravitational pressure anyway because some of the original work was a bit dodgy.