Computer definitely, not that it's hard at all for me.
With Legos, you get an exact step-by-step instruction sheet complete with intermediate pictures. All piecies have been 100% tested to fit together and function exactly as intended.
With Computers, you get a manual translated from chinese, with schematics that mean almost nothing. All pieces come from different vendors and nothing is 100% tested to work together. Different motherboards fit different cases, require different power supplies, support different cpu's and memory. Clips, snaps, pins, screws don't always fit. Everything either needs jumpered or configured. Parts don't always work when you get them, then you have to troubleshoot which part is stopping post. Then once you have a successful CMOS test, you have to pick out your flavor of OS... and that's a whole'nother world...
Now I have nostalgia for my old lego set... little brother, oh why did you have to lose all my good pieces...