The 787 would be too high a risk at this point, since it is not yet in production. The 767 tanker does exist, albeit in a different configuration. While some people say the new configuration is a high risk, Boeing has shown the ability to "cut & paste" wings, fuselage, cockpit, and software in the 737 series.
Another part of the decision that bodes well for those who use it is that less cost per airframe means more hoses in the air. That is what limits the tactical utility of the airborne tanker in these days, NOT the total amount of fuel available. There is only so much fuel flow available to tactical jets, and there are not a lot of B-52s needing tankers any more...