Assuming you're reasonably average in feeling the cold or not, I'd go for the 500.
I use an ME Lightline which has 550 grammes of filling as a year-round bag. The down is slightly better and the shell is a bit more technical so it's warmer, but having said that I've used it in a snow cave where it was about -10 inside (didn't have a look at the thermometer, but a bottle of juice was frozen completely solid).
Regarding Cathy's point, I've had -7 C at Dunkeld in April, just a very clear sky overnight and cold air drain in to the glen, but... on the odd cold night you shiver a bit and/or put on all your spare clothes, or you can cart around extra weight and bulk every time you go out with a bag. I've done it both ways in the past and having tried overkill and all right for most things most of the time I've gone with the latter. Or you'll need a bunker class tent too, on the grounds you can get a hefty gale any time of year...
Pete.
