You've rectified it somewhat in the second attempt but the important thing with stretch and squash is to maintain volume. It does depend on the style but generally you want to imagine whatever you're animating as a very elastic version of itself and what shape that would take when you stretch or squash it. Right now I would say you stretch it a little too thin.
There is a different technique called a smear which is more like what you're doing in the first version, however you would usually use that for very fast, long movements.
I always thought practicing stretch and squash on a ball was a bit of a tricky one since balls are actually elastic. With that in mind I would personally not employ any stretch on the way down, only on the way back up. Otherwise it looks like the ball is reaching out to touch the ground, rather than mindlessly bouncing.
The overall vertical motion looks really good. Definitely do as many of these types of exercises as you can find. If you can remember to employ spacing and timing this methodically in more complicated character animation will have a much easier time