Both are very fast, it seems that OA 5.1 BETA has been further optimized and is faster now, but Outpost may be just a bit ahead in terms of outright speed and responsiveness.
OA has always had non-existing entries marked in gray in its lists, plus you can filter the deleted applications, so removing these has always been easy.
Agnitum introduced similar functionality [Purge in Settings-> Application Rules] in 7.5, a welcome addition, because you had to manually clean non-existing applications before.
Both OA and OF have the functionality to automatically create rules for well known applications. OF is calling these "presets" [for example, "TuneUp", "Opera Browser"], which are updated through ImproveNet. OA works similarly in auto mode, protocols and ports are set accordingly, based on normal usage. Alternatively, rules can be created manually.
Both support IP blocklists.
Your system is stealthed by default by both applications. OF has an Attack Detection functionality, where you can specify attacks to block, specific ports you want monitored closely [including port weighing], ARP cache and various Ethernet/Wi-Fi attack settings.
Another good feature of OF is the Firewall Policy, where you can select from Block All, Block Most [only existing rules are applied, any other activity is blocked without prompting], Rules Wizard, Allow Most and Allow All.
Both HIPS are very fast and powerful, and generally are considered among the best in the field.
OA HIPS is more encapsulated and treated as one complete feature, with the only exceptions being the File and Registry Guard, added in 5.0. OA's perceived simplicity is actually misleading a bit - right-click on any entry in the list and you get a popup with a lot of additional/advanced settings, like a list of allowed/blocked actions, even the ability to set CPU affinity.
With OF on the other hand, you have more control over the separate components - Anti-Leak, System and Application Guard, File and Folder Lock, Removable Media Protection. Each of these can be turned on/off and settings separately customized, according to your preferences.
OA prompts have always been very informative in displaying details about parent processes, color-coded depending on the severity of the attempted action. They're continuously improving their whitelists, and nowadays it's very rare OA to block a driver on restart after installation.
Agnitum introduced improved prompts in 7.5, with more detailed information about the parent processes of the action in question. Also, the prompts now have a different colored header, again, based on the actions' severity.
A great feature in OA is "Run Safer" - applications are executed with LUA privileges. Great for browsers, PDF readers, media players.
An updated Online Armor 5.1 beta build is available:
Fixed an issue that could cause a massive boot delay or freezes on some systems.
False positives with Roboform have been fixed.
Fixed an issue where settings for blocked applications weren't saved correctly.
Opera should no longer be detected as an installer.