I ran CCleaner, Avira, and AVG, one by one, and recorded their findings. No changes were made in the PC from start to the end of these tests. Everything being equal, three tools were given identical environment. Yet results were surprisingly different. Which one to trust is the question?
Avira - Registry cleaner 2,307 and Junk file cleaner 51,931
AVG - Registry cleaner 126 and Junk file cleaner 20,525
they all do things differently, covering areas based on internal settings, company based algorithms, some clean deeply, others have a lighter touch.
in my experience, the actual cleaning aspects of the free versus paid-for versions are the same.
as to which one to trust, that is far too objective to answer. one program will work for one user but not another, that's the reason there are so many of these programs around.
personally, for crap removal, having tried a few over the years, CCleaner is my go-to program. for registry cleaning, CC is one one the better, non-aggressive cleaners but still not without potential dangers.