If your primary reason for going to the XT is for better digital decoding, then you may be disappointed. In my case, I think the decoding of the P25 Phoenix, AZ simulcast is actually a little worse on the XT than it is on the T. The P25 decoding on the mixed digital/analog non simulcast Maricopa County system seems slightly better on the XT than the T.
Considering Uniden has had about five years to refine the code, the processors ae now much faster, and the XT has a lot more memory to run larger programs, I was expecting a noticable difference in the quality of the decoding. Sorry to say, it just is not there. The XT still has that wavey digital sound and clipping of words. Compounding this is the lack of documentation from Uniden with instructions on how to fine tune the decoding parameters.
On the other hand, Uniden did make a good effort to fix many of the usability issues, so the functionality has improved. Also, it appears to me that the reception is somewhat better on the XT as compared to the older T generation. I would guess it to be about a 10% improvement. One other thing to keep in mind is that the XT is more likely to see firmware improvement than the discontinued T.
All things being equal, I would take the XT over the T. So, I guess it comes down to how much you will have to pay for the XT and what price you can get for the T.