I did mine in two - on the same day with a break. No preference really - just thought I'd chip in since all the other posters did it in one.
Having done several dual land-aways with a cup of coffee and a chat in between legs, it felt quite natural actually.
I take vancouv's point that you may have more chances of screwing somthing up on a two-part test, but on the other hand, you also have the opportunity to put right any mistakes and show the examiner you really do know what your doing so it probably evens out in the end. We'd done the circuit work on the first part, so after the nav and diversion, the examiner took over and flew us home while I relaxed - I was absolutley knackered after 3 hours in the air.