The major difference between these two laws is that the later law set up an enforcement system that seemed to make it much easier for fugitive slaves to be returned to their owners.
The later act set up a system of federal commissioners who were to hear cases involving alleged fugitive slaves. There would be no jury trials and the accused fugitive was not allowed to testify. The pay the commissioners got was doubled if they found the accused was a fugitive slave and ordered him/her returned. US marshals were required to enforce the law and they had the right to force citizens to help them to do so.
In these ways, the later law appeared to be much more powerful and much more likely to result in fugitive slaves (and perhaps free blacks) being sent back into slavery.