In reply to Nick Smith - UKC: I really enjoyed the RAB mountain marathon - it's a score event, so you can pick the course that you run and how ambitious you want to be in terms of accruing points (the Short Score of the OMM is similar, but at a time of year prone to worse weather, obviously!). However, this does involve more planning and being a bit strategic.
A course based class/event (i.e. the Saunders, the non-score classes of the OMM and the LAMM) might help you feel a bit more secure in that respect, as there will be other people running the same way as you (though this can go horribly wrong and you could end up following people on a different course!!).
The shorter courses on the Saunders tend lend themselves to following footpath type features quite a bit of the time - you might do more height gain than a 'similar' course on the OMM, but the nav and terrain would probably be easier.
Being able to grit your teeth and carry on when the weather's minging (or even too hot!!) seems to be the most important factor in any case!