I believe that what ever the highest supported connection mode for your device is, Say your Laptop, if there is a broadcast going on at G and N modes, the device should connect at the N mode and the speeds will depend upon the distance, any interference an the quality of the signal will dictate the actual file xfer or traffic speeds being sent over WiFi. You have to understand that the WiFi modes you connect at do not reflect the actual traffic speeds you will get. So if your set up for Single mode G, your connecting at 54Mb however the actual traffic speeds will be lower. Same goes for single mode N or Mixed G and N and the other modes. Single mode N you'll connect at 270-300Mb however actual traffic speeds are not the same.
I did some comparisons last year and posted them on here. Not sure where there at now.
I believe that if your connecting at the N mode while the router is running Mixed G and N, your device should connect at the N mode and that connection should be at 130Mps. The actual traffic speed will just depend on how the environment is set up.