It may be partly explained by the intensive use that FGW▸ 's stock gets, compared to the SWT▸ DMUs▸ . SWT has plenty of spare units so they spend a lot more time hanging around Salisbury depot being cossetted than your average FGW 158!
Ah ! another FGW apologist (employee ?)

I think this is an entirely valid point. The average FGW DMU set is pretty grubby externally whereas the SWT unit are alway very good. I see many going through Warminster and the difference is striking.
Set against this, I must say that FGW do make more of an effort to keep the interior of their trains tidier with regular walk-through rubbish collections.