I'm using both to monitor the same single IP address and they've picked up and notified me of listings appropriately. Now that my one blocklisting issue is resolved, they're sending me a periodic notification to let me know that my IP address isn't listed on any DNSBL. If I had to choose, I'd say Debouncer is perhaps the better of the two tools. It seemed to email me of listing (and de-listing) notifications quicker than RBL Tracker did. But both got the job done adequately.
I don't know much about the company behind either, so I can't speak to that. But I've been happy with the service provided by both.
And finally, I know a lot of ESPs and deliverability tracking services offer DNSBL monitoring of their own. Nothing wrong with those tools, but I was thinking it also couldn't hurt to enable one of these third party DNSBL monitoring solutions as well. Belt and suspenders, etc.