I think they both limit bandwidth, but the QoS does it by applying rules to the connection to classify the traffic. When I went to a VoIP phone, I turned on QoS to see if it would help maintain good phone performance. Not sure if it did that, but I noticed the limited bandwidth accidentally when I tried a speed test and noticed that I wasn't getting my expected speed. The speed test was classified as a "file transfer" and was given only 60% of available bandwidth.
Bandwidth Limiter (I'm guessing here -- haven't used it) seems to just set hard speed minimums and maximums for IP or MAC addresses. Useful for limiting a bandwidth hog that you can identify by address.