The core difference: PNG is lossless, JPG is lossy.
This results in around 3-4 times bigger file sizes on a PNG file on a quality level (9/10) without perceptible quality difference.
The drawback is that each JPG needs to be decompressed "live" from 24 to 30 times a second. On the other hand, the PNG method needs to load 4 times more data.
I only tested the performance on an Xperia Arc (1*1GHz), and I could notice some lag on the JPG. My guess, though, is that on any modern device (dual core +), there wouldn't be a noticeable difference.
If so, I would prefer the 17 MB (JPG) to the 77 MB (PNG) on a HD animation.
Has anyone tested this?
PNG supports alpha, JPG doesn't. No need for alpha in a boot animation. Both are 8 bit.
JPG is good for photography and "natural" imagery, but bad at hard edges and color fields.