The problem is, how often are you going to find two programs that are exactly identical, only one is open source and the other isn't? Personally, I like OpenOffice more than MS Office--but it's almost entirely because OO is free for me to use. Same goes for Gimp vs. Photoshop, but I'm not exactly a graphic designer, so I use the more basic features. 7-Zip lacks many of the features that WinRAR has, so I use WinRAR. If they were features I didn't care about, I might use 7-Zip.
So, the main thing is features. After that is price. If the features are the same, I'll take whatever costs less. If they have the same features and both cost the same, my brain will go into an infinite recursion, causing my head to explode in a fine mist of neural tissue and blood with the occasional skull fragment.