Very basically, words ending in "-los" mean that something that should be there is missing , which is negative (at least in case the absent thing is something you would normally like to have...).
Words ending in "-frei" have a notion of something being absent, this is not necessarily a negative fact, but normally either neutral or even positive:
wertlos - "Wert" (Value) is missing, the thing is not worth anything
wertfrei - "Wert" (value) is absent, which means "unbiased", so there is no valuing bias or prejudice in a statement
arbeitslos - "unemployed" the possibility to work and make a living is missing
arbeitsfrei - "not obliged to work, free" the obligation to work on an
"arbeitsfrei" day is absent
zwecklos - of no purpose - futile
zweckfrei - without following purpose - like childplay (no negative notion) or basic research without specific targets.
The notion is also used in marketing: Even if something that is kostenlos doesn't cost anything, it might also considered not to be worth anything. So marketing would rather say kostenfrei. The same applies for terms like fettfrei, laktosefrei or frei von Schadstoffen, which are generally preferred to the -los suffix by marketing because of their rather more positive notion.
The above applies to words that occur with both suffixes. There are, however, quite a number of words that would only occur with one of them. For this class of words, I would rather not say "positive" or "negative" - Here both -los and -frei suffixes would simply denote "not there".
- grenzenlos - limitless
- schadlos - free of (mainly financial) damage, but
- schadensfrei - without any (mainly physical) damage
- endlos - without end