Here's my 100% objective and correct analysis of the differences between Hughes and Kakko:
Skating: I think it's clear Hughes is the better skater overall, mainly due to his amazing top speed. Top speed also happens to be Kakko's weakness. Both are extremely agile, though.
Playmaking/vision/etc: I'd give this to Hughes as well. Both have displayed great vision and passing, but I think Hughes has a bit more creativity and is able to process his options quicker at high speeds.
Goalscoring: This one goes to Kakko, as he has a better shot and is able to force his way into dangerous areas.
Hands: basically a tie. They use them a bit differently due to having very different playing styles, but both have amazing hands.
Physical attributes: I think this is the biggest difference between the two along with skating. Kakko is a monster not just among his peers, but among pro players as well. This is also the tallest hurdle for Hughes to clear. I watched the U20s and U18s, and he is weaker than his average peer. Naturally he will grow, but it is unlikely he will outgrow his peers, and thus he will always have to punch up.
Overall: I can't really confidently say which one will be better in the future. If I had to pick one to play a playoff game for my team today, I would obviously pick Kakko. But 5 years in the future? How much stronger is Hughes, how well has his game adapted to the NHL level? Has Kakko's top speed improved, has his strength plateaued? I don't want to sound like some enlightened centrist here, but neither is a perfect prospect, and I think they are too close currently to make a confident decision. I would happily take the 2nd pick this year.