I wouldn't fit a combi unless i had a reason, ie freeing up space or maybe cost as for a new build, a combi is easier to fit than a system boiler, it is not that they are bad at what they do, its that they're different to system boiler which heats a cylinder, a system boiler is more simplistic and can be less susceptible to dirty system water, combis have plate heat exchangers that have very small water ways that get easily blocked, effecting hot water performance, if you loss hot water you don't have a hot water back as you do with cylinder and a immersion, unless you are clever enough to fit a electric shower, flow rates can be a problem in a big house, the more taps you open the more the water flow can drop off,
.I would also go for a condensing boiler system or combi, Worcester Bosch and viessmann make good boilers, and i think viessmann give a five year guarantee.
Some installer are fitting combis and cylinders together , they do this by running the kitchen off a combi and the rest of the house of a cylinder, the cylinder needs to fed by a tank as they cannot handle mains pressure unless you fit a unvented cylinder which is a whole other story.