Yeah I'm not sure either. Actually this is one of the few things I didn't really spend much time comparison shopping for either. Saw a good deal at my local hill pro shop, looked online to see if the price was right, and pulled the trigger.
I bought mine due to a few bad rotator cuff injuries, but it's helped for other things too. I say get the best stuff you're comfortable with wearing. I might look for shorts this spring too as I've had a few ass sliders that stung for a run or two this year!