Upload & Sell: Off
I think if you need to be discreet, a DSLR doesn't really cut it...
Think about it - with a desirable zoom lens attached, would the D5100 body make much of a difference compared with your D300 w/o grip? Not really. (yes, it will weigh less, but not any less discreet)
If you don't want zoom, the same thing still applies... the difference between the D5100 and D300 widens, but still doesn't compare to something in the mirrorless category.
I'd say either stick with your D300 w/o grip (easy enough to remove), or as Red Rover said, explore the Nikon 1 series, or other smaller-than-DSLR alternative.
(of course, do what feels best for you!)