I own a 925 and a 8X (both unlocked). It depends on your usage. The camera and the more frequency in which Nokia updates its apps shows up as its strengths. For the 8X, the audio is still better due to the built in amp - if you toggle off Dobly (925) & Beats (8X), the sound to my ear on the 8X is better.
Otherwise, designs (the best 2 designed WP8 phones available), weight, chipset, RAM, storage & screens are a wash. The only thing is that it looks like the 925 will get the 3rd row option and maybe the 8X won't with 8.1.
For t-mobile versions, you get faster data speeds because their 8X is just 4G and the 925's LTE bands are good.
So ultimately you're trading a more pocket friendly model and better audio for an improved camera, better OEM specific app and OS add-on support, faster data speeds. It's an upgrade, but not as big of an upgrade as of you were on VZW and going from the 8X to the Icon. Still, the 925 is a worthy 8X replacement.