I build my own PCs, so I've no personal experience of either of these other than as parts suppliers. Bearing that in mind you could try quietpc.com (especially if low acoustic noise from the PC is a requirement) or scan.co.uk.
Personally I've always found the best (sometimes only) way to get the combination of components I want for the peculiar requirements of a DAW is to assemble the PC myself.
However if low acoustic noise isn't so much of an issue pretty much any decently powerful i7 based PC should do the job, just add the extra drives and a suitable firewire card yourself if necessary.