Both are spectacular units, the HX 750 was actually rated 80+ Gold by Ecos but corsair sold and marketed it as 80+ silver because it couldnt quite do Gold at 50C, they are both great units with similar efficiency, modular cables, and great performance, i would go for which ever one happens to be cheaper if there is a significant difference in price.
I too thought the HX and TX were identical at one time, however, they are actually different platforms, the HX 750 and HX 850 are CWT DSG while the 750/850 TX are CWT PSH, the TX-C is CWT PSH II, and the TX-V2 are Seasonic built, just a heads up
With that small of a price difference its a difficult choice, normally i would go for the cheaper one but $15 more ontop of a $130 PSU isnt much so it makes the AX 750s fully modular cables a bit more tempting, its really up to you both are solid picks and about even.
Thanks for the advice guys just got the ax750 awhile ago, its true that I had a hard time choosing between the two. $15 difference for getting the top of the line psu really tempted me! Gonna put this on my rig tom. after work and hopefully everything goes well! Fingers crossed!