Between those two I would take the M6400 – its a rock solid tank of a machine (I used to support these directly at work).
The support on the M6400 will be far better as well – you *need* to pay the extra for 3 years warranty at the cost level of this machine, perhaps even get the full 5 years and if you can upgrade to pro-support. XPS support can be patchy…
Also it will take two hard drives – and do raid 0 or 1.
The battery life on the M6400 is not stunning – however its specification is insane.
I cannot praise the M6400 enough. Seriously.
(emailed you as well rob)