This has been discussed at some length in
this thread. I'd say that when reading on a 505 there are advantages and disadvantages to both and a lot of it is down to personal preference, but you'll see them both discussed in the thread I've linked.
There's also the issue of transferable formats, which seems to be a reason for choosing epub, but with software like calibre many books can be converted to other formats.
As you're based in the UK, you'll find that UK based shops generally offer epub for sony readers rather than lrf.