There is no legal requirement for logbook formats under CAA or FAA.
However, much depends on whether you are chasing other ratings etc, in which case you will save yourself a LOT of hassle if you log things properly, and this really applies to the FAA system which makes a distinction between all different kinds of flights. In my FAA logbook (Jepp I think) I have about 10 columns; logging x/c time for each flight between different places more than 50nm apart, etc. There are columns for instrument approaches (so one can easily check the IFR currency, etc).
When you apply for an FAA license(certificate!)/rating you need to fill in a form with a very detailed breakdown of different times. For example for my FAA CPL I have to provide the complex-aircraft time and even this is split between dual and PIC. OK, for this one I will have to wade through the logbook and add it all up, but you get the message I am sure...
I would use an FAA-style logbook for all your flights; it easily meets the CAA requirements, and all your FAA endorsements end up in there too. Crucially, FAA accepts JAA training but not the other way around, which is why doing it this way round is better.
Also, photograph the logbook occassionally (digital) and keep the pics, in case it gets lost
