well, i can play hacked hacked songs on expert (do a youtube search for one day remains by alterbridge), and have great difficulty playing a real guitar (damn fat fingers) so i personally think that guitar hero is worlds easier then a real guitar. people who have been playing guitar for years may tell you the opposite, but i feel my opinion is a little less bias as i was able to pick up guitar hero and get to hard, while drunk, in about 3 or 4 times playing, while real guitar, i pick up on and off and absolutely suck (i can only play some painfully easy solo by flyleaf, and a few random bits and pieces of other various bands)
as far as moving from medium to hard, try playing heart shaped box. that was the first song i 5 starred on hard, i personally found that song pretty easy. also, you can try going into practice mode and slowing the songs down. this will give you the ability to play with the extra notes, but at the same speed that you did on medium. worst case scenario, just pick a song in the first tier, and play it 5 or 6 times. you'll notice that you get further and further each time you play. also, try saving your star power for the solo section. you won't get big scores that way, but it will keep you alive through that section while you get better at the game.
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also, hammer ons and pull offs are you friends. check the in game tutorial for more info (you'll get an achievement for checking the tutorial anyways).