I think the main difference is price. If I bought 15 lbs of BRS ROX for around $250 I'd pay about $16/lbs. Or "Granulated Activated Carbon 25lbs/$225 That is only $9/lb Free shipping CANADA WIDE"
http://caco3reef.com/activated-carbon
It also has a larger surface area: 1225 m2/g for BRS ROX and 1670 mg2/g for his.