I find 8GB cards are generally big enough to handle a day of shooting, but I prefer a 16 GB card for a little more room. It strikes a nice balance between price and having enough space for a day of shooting. I don't see a lot of need to go larger on the D300s.
I also have a D800E and a D600. On the D800E with the larger file size I use a 32GB CF card - and it's plenty. I have a 32 GB SD card as backup on my D800E, but just use 16 GB SD cards on my D600 and V1. If you are shooting video, you can burn through a large card in a relatively short time so large cards are much more important.
The vast majority of card problems are related to user error. There are some problems with counterfeits, but lost cards, damaged cards, download mistakes, etc are by far the more frequent problems. A single large card - but not too large - reduces that risk but increases the loss if there is a mistake. I don't place a lot of weight on this issue since it is so unlikely and instead emphasize a balance between speed and price by buying one level behind the fastest card and a size below the largest card.