Running Port Sentry with packet capturing on a high load network will result in dropped packets. I've tested Port Sentry with packet capturing enabled and it seems that nmap with certain stealth scan options still easily elude it. This is a poor piece of software and I can't help but think that its only reasons for success are because of the placebo that it proactively responds to scans and the fact that it was written by a security company. It is the best open source program of this type that I am aware of. Port scan detection is a subset of NIDS so one can rightfully assume that snort handles this as well.
Snort is portable across multiple platforms using the libpcap library.
Because it is entirely a packet capture NIDS, it doesn't suffer from Port Sentry's port binding and is able to see a lot more of what's going on underneath. Snort can output in tcpdump format letting you analyse snort logs with many of the already existing programs that interpret tcpdumps. Snort can be configured to detect specific exploits going through your network which a program like Port Sentry can not. Software like snort can also analyse packets looking for specific exploits, and logging the session. This is good from an evidence point of view, or simply detecting what specific exploits are being launched against you.