Well they are like 2 different flavors of sound but I would definitely give the nod to the X3s overall ability..The amp section of the X3 helps tremendously with more power hungry phones. The Z is quite not powerful enough. I would say it works with about 80% of my phones with no issues but some of them I am pushing max volume. The X3 has no problems pushing 100% of what I own with ease and that is a big plus..I used my Z ever since it came out. I know that sound really well and still love that sound actually..The X3 has a fuller more spacious deeper mid section and even a stronger bass end over the Z..I find the highs sound more spacious as well. The Zs stage is excellent but the X3 has showed me the added depth it has over the Z which adds to the 3D like imagery Wolfson dacs are known for..If I do have a complaint I would say the X3 needs a more wider presentation of sound but for what it has it is a roomy sound and dimentiality akin to listening to a full end system on the go. That is completely worth it for that presentation alone imo.
Now if I throw on my 2stepdance amp on my Z then we are talking about bringing the sound of the Z to another level similar to the X3 by itself. But the whole idea of players like the X3 DX50 is so you don't have to have another slab of beef on the side of your order..