Orpington do Sevenoaks to Blackfriars trips, and Orpington to Kentish Town, which are shorter journeys than Gillingham's Rainham to Luton trips.*
If you'd like to stretch your legs more often, then consider Orpington.
A couple of years ago both had a similar range of start times (earliest at both depots was 03-something).
Gillingham there's about 60 drivers there. Orpington I'd take an educated guess at 45-48.
Gillingham did have nights then, but less than Orpington spread over more drivers. Night diagrams at both didn't look to bad as far as work content was concerned.
*Saying that, some Gillingham diagrams were Gillingham to Rainham and back a couple of times, and then to London Bridge, and then back with a different train, which would give plenty of leg stretching and tea making opportunities! It's just the main trip to Luton thats a long time in the chair.
They are both 'all stations' type work, and both have the same rest day pattern as described above.