I think your form looks much better in the 2nd video. I don't see your back rounding, looks like you're trying extra hard to keep it straight. I think you should just sit back a little more before you start your pull. When you start with your torso near-parallel to the ground you have to straighten out during the pull. If you start the pull with your head and chest up from the start, all you really need to do is break the ground and push your hips through.
It is much more obvious at heavier weights. Post a video of a near-max attempt so we can get a better gauge where your weak points are.