Honestly, I believe that ordering sample prints is not the way to decide between the two printers.
The two printer's output is too close, and there are too many variables to influence the look of any limited sample of printouts produced by a third party, for this to be of any help.
BTW, I get very neutral BW from the 3800, both through Advanced BW and RGB printing. People have been getting neutral BW from K3 inks for quite a long time now for this to be an issue.
Myself, being a conservative sort of person when it comes to these things, I would not yet invest on a Canon or HP pigment printer, until there is a bit more collective experience out there from people much more skilled to judge things than me. But then, that's just me.