And an old B/W video that shows through-canopy ejection trials. as is plainly visible, the canopy shatters before the pilot actually begins to emerge through the canopy itself. At 0:13 of the video, the canopy opens up almost like a clamshell, as described in the other document I'd posted.Youtube video
The only problem with thru the canopy ejection is that when the canopy shatters, it should be absolutely ensured that the flying shards are not razor sharp jagged daggers that you would get when a window pane disintegrates or a mirror disintegrates. Or else the pilot will get skewered, defeating the point of ejection in the first place.
It is for that reason that they have safety glass mandatory in automobiles, which is basically two tempered glass sheets sandwiched together on a thin film material, so that when it fractures in an accident, it sort of holds together
and even if shatters, it shatters into small pieces with rounded edges and dont kill the passengers inside when they are showered.
Maybe the plastic material used in the canopies dont produce that kind of knife like shards that glass does.
But between jettisoning the canopy and punching through the canopy, the latter is definitely faster , while the former will take a few tenths of a second for the canopy to get clear of the plane safely before the seat can come out and the through the canopy system is probably safer as well because there is no canopy flying around that can impact the pilot(s) ejecting.