Damien,
I think the general consensus is that the Omni-directional is only usable in strong signal areas, where there is little or no interfering signals (multi-path, reflections, etc.)
A directional aerial will always be better at capturing more signal and ignoring interference from other directions, but of course the directional needs to be aligned with the transmitter.
How much better? Depends on the model you upgrade with, the sensitivity of your TV, etc.
Robert