On Thursday I finally managed to join the Southern Queue for the first time, and I did again on Friday. Now that I've experienced both queues I will try to compare them so you can make your choice based on it.
Northern Queue from Southfields
- short walk from Southfields station (10 mins)
- "refreshments" available (drinks, burgers, etc.)
- more people around giving you free stuff (Yoghurt!)
- a bit more atmosphere
- longer (average wait until entering the grounds was about 2 hours 30 on average)
Southern Queue from Wimbledon
- shorter (waited two hours on Thursday when Henman was playing and only 90 minutes on Friday when I arrived at 9 am and was in when grounds opened)
- longer walk (20 mins from Wimbledon Park station, about same from Wimbledon station)
- less refreshments (not that I use them much)
- no yoghurt
In my view, if you are somewhat late you should join the Southern Queue. Even with the longer walk you will be in earlier.
Since I don't queue overnight I can't comment on that, but I'm pretty sure chances to get show court tickets are better there too.
The Northern Queue is more entertaining overall being at the road so you should definitely join it at least once.