Considering the inclusion of LJStreamUD with the LabJack U3, that would certainly mean a quicker time-to-solution with LabJack U3 than with Arduino Uno. LJStreamUD is an all-inclusive data acquisition software package, that will automatically create a tab-delimited ASCII file of your data. With Arduino Uno you would need to develop this data handling on your own.
As for project support, LabJack has a very active support forum at http://forums.labjack.com/index.php?showforum=16
, and of course Arduino has this forum. I guess it boils down to deciding if you want a basically plug-and-play data acquisition application, or if you prefer to write your own.