If you are comfortable with C then it will likely be more compact and faster. LUA projects barely fit in the available RAM so only rather simple apps will do.
However, LUA, effectively being an interpreter, is much easier to experiment with. Using the default speed setting (9600 baud) flashing a moderate LUA app takes longer than flashing the firmware, and "make"ing the app is likely to be just a second or two.
My experience with using LUA so far is that I see myself moving to C/SDK as I am tired of shoehorning my program into the restricted LUA environment. C and LUA both have a place.
YMMV