From what I hear they're either the same screen or the 710T is a slightly newer panel (and hence the slightly higher ratios). Both are 12ms.
The 172x was popular because of it's snazzy stand and fold-up capabilities, but I think that the 710T is a slightly better panel (although it would be an extremely marginal difference).
For what it's worth, the 710T used to be about $50 cheaper than the 172x when the 710T first came out (so the 172x had dropped in price considerably and I think it is being phased out).