The two are so close you might as well ask yourself a different question: do I want modular or non modular, and do I need that extra 10A @ 12V offered by the 750?
Both are of equal or similar quality, with the HX being a Seasonic OEM, the TX by CWT. As far as sound goes, both are quiet, although the 750TX may have a slight edge since it's fan is bigger (135mm vs 120mm), but I don't really know.
@above - exactly. Or another way, what will the PSU be powering? In all likelyhood the HX will be more than sufficient. I just got the TX since it was on sale a while back @ $100 AR/tax/shipped.
This rebate seems to recycle at least for the last 3 months, but dont worry if you dont bite on the zipzoomfly deal its about $5-$10 difference at clubit and buy.com. I've been watching the 750 corsair one for last 3 months weekly. Good chance it recycles again at zipzoomfly.