most installers now will opt for the combi as they are eaiser to install,nowadays people jsut want instant h/w my old adage is if the boiler breaks down(yes new ones break too )you have no back up,at least with the cylinder & immersion you can have some hot water and put another jumper on to warm up,
if you need the space where the cylinder is and the loft space where the jockey tank/cold water tank is then go for the combi option.
the vaillant is still the best but being caught by worcester look at the gloworm cxi which is the same as the vaillant only different badge although they wont admit it!
Hope this sheds some knowledge let me know if you need more info
T