Just boils down to what your priority is with a chain oiler you will get a lot more mileage from you chain and sprockets and have the back end of the bike cover is scht or do you go for less mileage and keep you bike nice and clean.
Of course the next question is what's the difference in mileage and from what I seen and read I'd say 10,000 + miles.
I prefer the clean bike and spend 150 quid( or less) every 18/20,000 but it's each to his/her own.