I voted links being harder, for me at least.
I grew up in hills and trees and consequently, parkland style golf. I would truly love to experience some links golf, but have not yet. I have played some flat courses with no trees, but they were mainly just flat treeless goat farms, nothing like the links courses you would find in the Northwest, Michigan, or of course the true links of GB. On the flat courses I have played, the lack of reference points or trees to frame my target has been a little disorienting, and even though they were "easier", I usually play better with trees.
I agree with Hardballs that links would transfer to parkland better than parkland to links. I think with links, you would really have to focus on a target, which you should anyway, but with parkland it is often framed for you.
I would certainly like the opportunity to prove or disprove my theory though!