Which is more efficient, a system boiler or a combi?

Sirbendy

Reason I ask is we're moving (again)..from a 3 bed semi to a 2 bed top-floor flat. I love this house, I love the garden, I love the garage...I do not love the £300 water bill, the solid ground floors with no insulation, and the fact that I've been reduced to living upstairs these last few weeks..heh.

So we've found a flat close by, saving £150 (and a bit)/pm in rent, halving the water costs, dropping council tax etc. However, this place now has a Potterton system 15/2 condenser boiler in it..nice unit, not been expensive until this cold snap.

The NEW place has a Potterton too...a Performa 28i non-condensing combi...listed as "for larger houses", not made since 2006. All radiators have TRVs. and the floor is heated to an extent by the flat underneath.

Which would people say would be cheaper? A possibly oversized D rated combi in a top floor flat, or a half the output A rated boiler with a hot water tank and a room 'stat with no functioning TRVs in a cooler location?

I'm curious, nothing more...we'll see in a month or so, I guess...

I'm thinking due to heat from downstairs, then showers and washing up apart the boiler may well be idling quite a bit, whereas this system boiler has a 'stat in the cold downstairs and seems to be fairly thrashing itself..