Steven,
To respond to your question, this is a pretty difficult one for me to answer accurately. I use more caution on this. I'll list a few pros and cons as to how I have come to this decision, and talk a little more after.

Pros to commercial:

1. There is usually a pretty strict policy as to what is available on a commercial product. This generally (and I use this term loosely) makes the product safer.
2. MOST of the time, the bugs are worked out before a commercial product is released. This is a very general thing too because the internet is always changing, so it's hard to have bugs worked out for things that haven't happened yet.

Cons to commercial:

1. It's generally going to cost some money, and depending on the product, it could get expensive
2. There will not be a lot of different things available to attach to the product or modify it, but anything that is available to modify the product to your needs is generally going to be safe because the company that puts its name on it is going to run it through some serious testing before they say that it's ok to use and modify. (if it's a reputable company that owns the product)
3. Sometimes the product won't do exactly what you want it to do, and for that reason, it could end up making you do more work, and this would make you want to modify the product (see #2!)

Pros to Open Source:

1. There is a ton of things available to you with Open Source
2. Open Source will not be as expensive (if there is any expense at all)
3. Open Source is easily modified for whatever needs you have

Cons to Open Source:

1. Although there are usually a ton of things available for Open Source products, you never know who exactly is making these things, or what kind of programming or other things that they are putting on your computer.
2. Open Source can contain bugs (just like commercial products) but can still be released, even with the bugs.
3. Modifications will be widely available, but again, you never know for sure what people are putting in the code, unless you know exactly what the code is doing, but then again, if you know it that well, why are you not making it yourself so that you know the code that well? (just a point to ponder)

With all of that being said, you can see that I have only come up with 2 pros and 3 cons (off the top of my head) for commercial, and 3 pros and 3 cons for Open Source (although 2 of the cons for Open Source are basically the same thing, just slightly different areas of Open Source) but the problem is, the few cons that I came up with are HUGE as far as my decision as to whether to use OS or Commercial.

Not all of these things are true for either of what I have just listed, so I guess you can say that I'm "stereotyping" which isn't exactly fair. An example of this is Joomla. While Joomla is Open Source, I have done some investigating and Joomla is very reputable, so in that case, I use it as it is safer than most Open Source products. My problem is that I know enough .ASP to be dangerous, and virtually no .PHP at all, meaning that I have no idea what exactly this code in Joomla is doing (since it's PHP and MYSQL based). There are others out there, but I don't believe that they are as reputable as Joomla, which means that I probably won't use them until I have heard otherwise.

Basically, I swing to the side of caution. It tends to be more expensive, and maybe causes me a lot of extra work, but I understand what the code is doing and I generally don't have to worry about what things are being put into my computer, or my customers computer.

That's my two cents. If you disagree, please tell me why. I'm very open minded to change if there is a good reason for it. An example of that is the FF vs. IE browsers. You made me think on a lot of different things there. :-)