They both are quality but the 850W would be a bit more comfortable with dual 6970s for sure.
6970 CF, with no OC, will draw a max of 450W (less under normal gaming loads)
GTX 570 SLI, with no OC, will draw a max of 500W (less under normal gaming loads)
Add 200W for the balance of the system and at 750W you are just not quite running with a comfortable margin...
So, best to get a bit more power.
As to the TX line, it's generally quite good if you are on a budget. Yes, the HX line is better and the AX line is the best.
The XFX PSU linked is built by Seasonic, as is the 750TX V2... but it is the better unit