To make it as simple as XPAC vs Cordura is not really accurate. To put it simply Cordura and XPAC both have different fabric weights. Most often, when people refer to Cordura they are referring to 500 Denier Cordura or 1000 Denier Cordura and when people refer to XPAC they are referring to a laminate applied to a face fabric .... however most often XPAC is applied to lighter weight face fabrics such as 70D or 210D. XPAC can be applied to heavier fabrics like 500D or even 1000D face fabrics.
We (Seekoutside.com) have used a lot of Cordura XPAC before .. it is possible to have both a Cordura face fabric and XPAC laminate. In fact, we have used it in a 330 Denier and 500 denier Cordura with an XPAC laminate and our Slate Gray we used to offer was a 500D / 1000 D Cordura face with XPAC laminate. . These are excellent fabrics for a mountain hunting pack. However, usage matters and they are overkill for a trail pack and underpowered for a canyoneering pack. Remember usage matters !!!
On lighter Denier fabrics there are many ways to add strength so it can get confusing (Like Dyneema reinforcements) . Leaving that aside for the sake of this discussion .. 500 Denier Cordura is great stuff. 500 Denier XPAC with a Cordura face is even better almost all the time. The reality is, most of the time a 210 Denier is fine for trail walking and bushwhacking. Our (Seekoutside.com) Olive fabric is a 420 Denier Xpac which is pretty close to 500 Denier fabrics .. and our pocketing is often 500 Denier Cordura. Essentially we often pair a 420 Denier main XPAC bag with 500 Denier Cordura (not XPAC laminate) pocketing. This results in a close to waterproof main bag, and pocketing that is highly abrasion resistant resulting in best of both worlds performance. If you have any questions, don't hesitate to contact our CS. We are strait shooters and have used both XPAC and Cordura a lot. In fact, the bulk of our suspension is Cordura.
Note: The term "waterpoof" used here depends on bag design, seam sealing and other factors. Outside of the "waterproofedness" of the fabric, there are many benefits such as lack of needed rain cover, quick dry time, and blood clean up. A well designed XPAC pack bag, will not require a rain cover, but may not be submersion proof for long periods due to seams and access points.
edited to clarify terms
Thanks