Chickencellar
you say vented cylinder (which is the traditional type fed from a cold-water tank) but perhaps you mean unvented? (the more modern, high-pressure type fed directly from the watermain?
Bearing in mind that a bath holds around 100 litres, and in winter you may need almost all hot water with very little cold, the minimum usable size for any cylinder is about 125 litres (for example, a very common size in older houses is 900mm high x 450mm diameter), which is nominally 117 litres)
But it is more usual, if you're getting a new, unvented cylinder, to fit larger. Usually between 200 and 300 litres. With the improved flow and pressure (subject to your incoming water supply and the size of your pipes) you can then run two good showers at the same time, or fill a bath quickly, or have taps that don't run hot and cold depending what other people in the house are doing.
The boiler will run more quietly than a combi, because it will rarely, if ever, need to run full blast, and most of the time it will be ticking over at modest speed when needed to maintain your radiators and cylinder at the temperature required. You may have noticed that a combi usually starts at max power every time you turn a hot tap on. The more people in the house, the greater the advantage.
Because the boiler runs less often, it is more economical in use of gas, and you are not frequently heating up the mass of boiler and pipework, then letting it cool down again every time. But the cost of hot water is very low, compared to heating a house in winter.
If your incoming water supply is poor, you can have an accumulator to give a limited amount of high-pressure water, or even an old-style vented cylinder and loft tank. This can fill a bath faster than a combi, but shower pressures will be rather weak unless you have a (somewhat noisy) shower pump.