I have some pretty basic needs, and they both work good for me. That being said, I find maintaining Blue Iris motion detection a lot easier because it's in one place (Blue Iris) whereas IVS has to be setup/maintained on each individual camera... that requires that the browser plug-in works correctly, which has always been a crap-shoot for me.
Others here have some pretty complicated motion detection needs. IMO, Blue Iris wins hands down for those.
If you haven't already found the awesome Blue Iris video tutorials that @Dasstrum made, definitely search for them and check out the Motion Detection one.
Also, the version of Blue Iris that came out a few days ago announced a new option coming in the future where it will be able to send a picture of the alert off to a 3rd-party service (of your choice) for further analysis, so there may be some cool things possible with that (down the road).