I would also suggest taking a look at your network switch/router. Many sold are still 10/100 and if you want maximum network speed with the Auralic you would need a gigabit switch. The sonos only has a 10/100 port on it, so no big deal.
Cat5e and Cat6 can both handle gigabit if installed correctly. Cat6 installed incorrectly (or patch cable manufactured incorrectly) can be worse than a good job with cat5e, so who installs it is almost more important than which cable you choose.
Al's proposal for testing is exactly what I do for my home installs. When I paid professionals to do it for work, part of their deliverables were the test reports.
Also, a last bit of advice for anyone else doing this is to run an extra cable or two. You never know when you may need it. I only ran two to my living room and am kicking myself. You could imagine a tv, home theater, xbox, dvd player, and sonos all needing an Ethernet connection. You could always put another network switch in the living room, but then you are splitting your gigabit home run amongst all of those items, not to mention eating up one more electrical outlet. Best case scenario is that only one is actually doing any serious transfer of data at once, but you've still added one more item in between your source and destination.
