Hi
I've never used a hawkeye but I do have a nanocom. I believe that the nanocom can do more than the hawkeye and is also cheaper.
I know that the nanocom can do more thing than the actual Land Rover computer. eg programming second hand keys withouy the bar code(ps Rover 75 keys work as well )
So I would say go for a nanocom you'll get lots of support from the forums on how to use it.