Both are excellent!
You have a ton of support here on RC tech for D812 set ups and Tessman himself comes on here to post tips and advice etc. Having just raced yesterday the best handling cars out on my beat to hell track was the new XB8 and the D812 and the laps times showed it. As usual parts support (wait the D812 doesn't break!) and cost should be factors...also with X-Ray it seems like this years car will be replaced by some other "wonderful new design" from Juraj next year - I think that potentially chaffs some people having to buy a new kit most years because in their eyes the old one is obsolete (whether that is really true is debatable because the old XB9 is still a great car) but I wouldn't let that dissuade you from the XB8 - it is a great car and that is coming from a HB guy... but if someone tells you it is more durable than a D812 they are lying - handling about par (from what I have seen) cost: obviously the D812 wins that one...