Audio “quality” is a very subjective subject. There are so many variables at play to simply say that a higher bitrate audio file “sounds better” than a lower bitrate one. What sounds good to me may not sound good to you, even if it is played back through the same player, in the same room, and through the same speakers for both you and I. To some extent you can reduce some aspects that influence the sound, such as the size of and furnishings in a room and the resultant reverberation or frequency absorption, but you will never eliminate external influences.
Even listening through headphones different people will have different perceptions of which track sounds better. Headphones sit on peoples’ heads differently, the headphone speakers may be closer to my ear canal than yours, and we have different sized outer ear canals. Speakers have a specific fundamental frequency or resonance. Expensive headphones do not always produce a flat frequency response.
In the same way that we have a “persistence of vision” , eg. look at a red light and then at a white wall and we usually have a green blur left over for a while, alternating between audio sources can confuse our brains into wrongly interpreting what we are hearing. The sample MP3s play back through your computer’s sound card and I wonder how many people listening were aware of how their software volume control, equalizer settings, and the hardware in general affected what they heard?
OK, so you would say that the effects would be applied equally to both audio sources, but bear in mind that some types of music tolerate audio compression better than others and so may make it harder to tell the difference. For what it is worth, the type of music in the clips sounded better to me at the lower bitrate.