The SuperB and XLR8 are not even the same class of machine, how can you be comparing them?
I got the XLR8 because it is significantly closer to professional grade with an extremely well designed downdraft system.
There really is no such thing as the "Panasonic Servo Package", they are standard feature of this machine.
Definitely spend the few extra bucks and get the Plasmamate software.
Nesting is up to you whether it is worth it, it is not cheap, but if you are always cutting multiples you need some sort of nest software.
I do have Sigmanest, I feel it does a great job of making nests. I also feel it is difficult to use, the settings are often hidden or not straightforward, the CAD tools do not adhere to age-old industry norms and non of the supplied tutorials worked (they didn't seem to care and offered paid training), other than that tech support has been very good.
Alas, I have no experience with sheetcam, because I received a special deal on Sigmanest so didn't have cause to shop around.