Their cleaner than any of Southern's trains!
In my extensive experience, that's really not true. I'd say that one is no worse than the other. I generally find that Southern trains have more "day-to-day" litter such as newspapers, but less ingrained dirt in visible areas (areas such as those under the seats are unfortunately a different matter). By contrast I usually see at least a bit less litter when I use Southeastern, but a lot more dirty seats/toilets, ingrained muck and disrepair of upholstery. However, both have significant cleanliness shortcomings, largely due to a shortage of time to clean at termini during the day, resulting in problems spiralling out of control, and also due to the bad habits of a number of passengers. In addition, of late I am finding that passengers on Southern mainline/coastal routes and the Oxted lines are far worse than Metro passengers at depositing unsightly litter, whereas it seems that most problems on the Southeastern network occur on the Metro routes, as stated so often up-thread.
I've done my best to tackle these problems with some of my rail industry colleagues, but it's proving really hard. Unless passengers do something significant to change their habits, maybe with the threat of being prosecuted for littering unnecessarily (Byelaws breaches relating to interfering with the comfort of other passengers?), there will be major cleaning challenges for years to come.