As far as I can tell, they are very different in their goals. Portmaster is targeted at security and business customers, it's surprisingly powerful for an open source project. The interception mechanism seems to be based on iptables, but I skimmed over the source code only quickly.

Little Snitch for Linux, on the other hand, is much less complex and tries to analyze and filter based on DNS names, not IP addresses where possible. It is not made for security, but rather to provide insight for the curious what's going on. It hooks into the kernel via eBPF, not iptables.