ZX450 v2 has more "headroom" than the ZX475ti. Which means it can output more voltage, and therefore more power, but is limited to short bursts, due to the size of its power supply.
ZX450 - more dynamic power
Ti500.4/ZX475ti - more reliable
You would be hard pressed to actually hear a difference between either of them. I went with the Ti AL series for cosmetic reasons (I think the Ti's look better, and the AL's don't rust).