When you say Outpost Free, do you mean the old stand-alone product, or the firewall in the free Security Suite?
The former has been discontinued a long time ago, and will only run on XP/Vista, not sure what OS you are running ATM.
The latter hasn't been updated for quite a while now, and if that's what you mean you may have to carry a bit of deadweight with the full suite being installed [at least the last time I used it more that 12 months ago], instead of just the firewall component.
I installed OA again on a machine, which has been OA free for over 2 years, because of excessive boot slowdowns, however the latest version is comparable with Outpost and Comodo, which both don't affect boot time. The once arduous Safety Wizard is very fast now, with a lot more applications recognised and trusted automatically, just one of the benefits from real-time cloud lookups with Emsisoft's Anti-Malware network. Chrome and Sandboxie compatibility has also been improved.
Latest Comodo Firewall is the most complete feature-wise. As if the firewall and HIPS aren't enough, Behavior Blocker, Virtual Kiosk, Sandboxing, Kill Switch are also included and are very good reasons to check Comodo. You'd need to master the new GUI, which I personally like a lot, although it could be a level or two too deep, compared to the easier OA interaction.
Both OA and Comodo feel light and responsive, the HIPS in both have matured enough not to be nuisance in everyday use.
Either way, the winner is you.