Overground you will easily get on, but you won't get a seat. It will get insane at Brockley though just hope you're not near the doors.

London Bridge train is fine. Probably won't get a seat but far less full.

Changing at Canada Water is pretty slow due to one escalator. Might have to wait for a couple of trains to get on the Jubilee but most days it's fine.

Changing at London Bridge is hit and miss. Sometimes it's empty and sometimes it's totally backed up and you will be waiting quite a while.

I just take whichever one comes first.

Coming home I recommend going to London Bridge. You get a seat. You won't get a seat on the Overground coming home from Canada Water.