The G5 seems to have a lead in most benchmarks. Both systems should support four threads.
Fairly comparable, but a PCIe G5 would have better expandability (even if it is only PCIe 1.0).

We have the added benefit of the support of newer graphic cards.
As the drivers for those improve we should have better video performance than the Mini.
Also, if Posideon were to be upgraded, we might be able to support USB 3.0 cards with the G5.
And of course, those options would be possible for the X5000 as well.

So, again, we don't come off too bad in a comparison like this.