I played Giana Sisters the wrong way when it came. It's really not that hard to advance to high levels if you just let go of your perfectionism
Apart from the superb music, I'd say SMB is superior in terms of level design, graphic design (!= number of colors on-screen, ffs!), and playability+addictiveness.
GS was an ace game, tried to play it today even
But the disk had finally become unreadable. Not whining, took 20 years
And yeah, I've hacked my gamepad to use (megadrive) button C in place of 'up'. Up to jump is horrid (c) 3po