I would think that taking off would be easier for Cadence since she's so much lighter. The heavier you are, the more power you need to get off the ground. I would think that Celestia would need a running take-off.
(It's also theorized pegasi have some kind of magic that cancels out the gravitational effect of their mass as well as the mass of anything attached to them, which is why the things they pull behind them stay horizontal without dropping down. Also, if you do out the physical force calculations for the Sonic Rainboom, you basically HAVE to assume a very small or zero mass in order to get a "survivable" magnitude of force.)
The little details. Oh, gods the little details. You manage to make it seem so much more real without compromising the show proportions. Fetlocks, muscle lines, bone protrusions, general shadings. I am a stickler for little details. Do the maths.