The Sapphire is arguably the better of the four listed. Powercolor is best price wise but its QA has been questionable (higher percentage of DOA's). To be honest the HIS and Powercolor are basically identical. Personal choice really, there's not allot in it, and given that your constrained to the lowest clock in CF the better cooling and OCing ability of the Sapphire & HIS are irrelevant if your other card is reference XFX.
And is it worth it? You will get a significant increase in FPS for gaming, better performance than a GTX580 or 6970 (single monitor only), so bang for your buck its certainly a worthwhile upgrade.
Hope that helps.