The primary difference is that Better has different aims. Unlike AdGuard, we are not focused on blocking ads, we are focused on blocking tracking. Although 99.99999% of ads on the web involve tracking, as they are behavioural ads.
We do allow ethical ads that do not track you, as they are a legitimate way to build a sustainable business. Unlike other ad blockers, we block all ads that track you, we do not have any “acceptable ads” where corporations can pay the ad blocker to be whitelisted.
Compared specifically to AdGuard and Little Snitch, Better functions differently because it doesn’t work on the network level, it works on the browser level. Better is not a firewall, it blocks inside Safari. Unlike Little Snitch, you do not need to do any configuration. We maintain and regularly update our own blocking rules to keep you protected. (Little Snitch works great the way it does as it gives you an added insight to network connections, @aral and I both use Little Snitch alongside Better.)
And of course, as @jelle said, we are tiny, independent and everything we build is free (as in freedom) and open
I hope that answers your question!