I’m at neither TOC, but it will very much depend on depot. Could you be more specific?
GTR (assuming you mean TL rather than Southern) is all DOO, some depots are intensive metro work (St Albans, Orpington), some are mostly mainline (Three Bridges, Bedford). Sundays are outside but you’ll have committed sundays (12 per year) which you’ll have to work if you can’t find cover for them. Pay is towards the top end.
Chiltern seem pretty well thought of generally and often recruit qualified drivers (whereas GTR tends to take on trainees, many of whom leave for better work). Pay is certainly less than GTR, but it’s a mix of DOO and guarded work, London Marylebone sign as far as Birmingham.
Swings and roundabouts.