Hi Marcus!
Should I go with EXT4? Or XFS with just the defaults? Or XFS with one or more tuned parameters? ... I'm not using quotas, and understand what inode64 does and am fine with that, but what I'm still unsure of for a VM environment is the delaylog and nobarrier options.
I've been using XFS for many years now and i strongly recommend it for anything besides /boot. Considering a virtual environment i would strongly suggest NOT using nobarrier (i.e. use barrier). You can run into big trouble should the system ever lose power. In fact the only time i ever managed to damage a XFS filesystem on all those years was inside a VM and with the nobarrier option on and the UPS died (and yes the server had a functioning BBS battery attached to the RAID). The delaylog option can be recommended hands-off, since it speeds up metadata operations considerably (up to 10 times faster!). And for your last quesion, stay with the defaults when doing mkfs.xfs, optimizing for stripe width and stipe size and all those other options really only make sense on a physical machine.
hth and good luck, Michael