I just went through the a similar decision, although the Series 8 was only one of the contenders in the field. I had to get a daily driver that can cop some of the major traffic laden sydney roads day in day out without being a pain in the ass in any way and most importantly considering the hours im going to spend in it, i have to want to get in it and spend time in it.
At the end of the day i'm a Nissan lad at heart so i may have been a tad biased but the slightly bigger interior in the whale, bigger boot and my experience with them before won the day over. I had a friend of mine that clocked up 300,000kms on his lightly modded GTR before there were any issue's with it engine wise.
I'd reccomend testing both before you pull the trigger, get comfy in them because for me personally, i know there will be days where i'm pissed off with a great number of things let alone traffic and having a vehicle that add's to it is just ludicrous.
For something completely irrelevant i looked at about 20-30 different vehicles and my top 5 ended up as the following: R33 GTR, FPV F6, E46 M3, STI, R34 GTT 4 door.