i don't think visiontek are on ati's hardware supported list.
http://www.ati.com/developer/hardwarepartners.html
visiontek is probably powered by ati and may be based on the reference card by ati.so will be a few differences(maybe subtle)
but also may use slower ram than the ati reference cards.so less overclocking for it.
were the ati one is built buy and could be of better quality & more stable.and uses faster ram modules so should overclock better.
anyhow thats what i think.