Yes, Online Armor is fine, it has some fundamental HIPS protection, I especially note direct disk access prompts. And I put two posts in on the GAOTD thread. The OA forums are kewl, plus Wilder's has lots of discussion.
One restriction on the Free that I noted was that the restricted ports were unchangeable. Plus backup stuff is I think lighter and easier on the Pro, you have to do some stuff by hand on the Free. Config flexibility, certain other techie features .. beyond port-by-port .. is Pro only. So it is certainly advantageous to get the Pro, and they have some sort of $10 - 2 yr. thing available too.
Not sure what all the differences are, there is a chart, I have not run the Pro and am not too firewall-savvy, suffice to say that the Free is very good, the Pro is superb. In the past I have used mostly Zone Alarm, Kerio / Sunbelt and I have Comodo ready to be deinstalled on a work puter. Today Outpost is the main solid freebie alternative to OA .. a recent addition to their line, PC Tools also possible. Generally I would bypass firewalls that come from anti-virus companies trying to expand to a suite. Unless you really like the Suite as a whole, which is a lot of eggs in one basket.
With OA you have a small, really competent company, I combine them with Avira anti-virus. I was a little surprised to see GAOTD (expected more likely Bits Du Jour).
So unless you are very happy with what you now have .. e.g. a suite that fits you fine or a long-term accommodation to Outpost, Kerio-Sunbelt, PCTools, crikety old Sygate .. a bit dated now .. or another .. I suggest this as the best to do.
Note: you want to put special attention to whether an existing firewall can be successfully turned off, or needs a full deinstall, and how to do a deinstall. I would google that for any firewall ready to be decommissioned.
In general, Wilders has a ton of info, and many there are OA simpatico.