What is the best hot water system?

Halitosis

We need to move our boiler and hot water cylinder (for a new extension), so have an opportunity to change from the existing gravity-fed vented system and wish to improve the pressure/flow from the showers.
Being a family of 4 with a need to run two showers simultaneously, I am led to believe that a combi-boiler would be unlikely to provide what we need. We dont mind spending a couple of thousand if it means we reduce bills through long-term efficiency...

Should we:

1. Install another gravity fed system plus a shower pump (or might a larger header tank increase pressure?)?
2. Change to an unvented system (no header tank or pump rqrd., our mains pressure appears to be good, but annual servicing essential)?
3. Go with a 'mains pressure hot water' vented system (less frequent maintenance but more expensive to install?)

Any experience or knowledgeable advice greatly received!