I try and stick to bottles where I can, unless the pub is a specialist craft beer/real ale pub. I find a lot of pubs aren't cleaning their lines properly or something, so when I have a beer from the taps it tastes bad. Bottles from these places are from larger craft breweries usually (Brewdog, Camden etc) and so have been pasteurised, filtered and are less likely to spoil.
When I'm somewhere like Craft Beer Co, Euston Tap, or one of London's many good beer places, I know their taps are good quality and well kept so will not differentiate between the two; I'll take whatever I fancy from the menu.